docs(bench press): add initial docs

modules/benchpress/docs/index.md

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+# Benchpress
+
+Benchpress is a framework for e2e performance tests.
+
+# Why?
+
+There are so called "micro benchmarks" that esentially use a stop watch in the browser to measure time
+(e.g. via `performance.now()`). This approach is limited to time, and in some cases memory
+(Chrome with special flags), as metric. It does not allow to measure:
+
+- rendering time: e.g. the time the browser spends to layout or paint elements. This can e.g. used to
+  test the performance impact of stylesheet changes.
+- garbage collection: e.g. how long the browser paused script execution, and how much memory was collected.
+  This can be used to stabilize script execution time, as garbage collection times are usually very
+  unpredictable. This data can also be used to measure and improve memory usage of applications,
+  as the garbage collection amount directly affects garbage collection time.
+- distinguish script execution time from waiting: e.g. to measure the client side only time that is spent
+  in a complex user interaction, ignoring backend calls.
+
+This kind of data is already available in the DevTools of modern browsers. However, there is no standard way to
+use those tools in an automated way to measure web app performance, especially not across platforms.
+
+Benchpress tries to fill this gap, i.e. allow to access all kinds of performance metrics in an automated way.
+
+
+# How it works
+
+Benchpress uses webdriver to read out the so called "performance log" of browsers. This contains all kinds of interesting
+data, e.g. when a script started/ended executing, gc started/ended, the browser painted something to the screen, ...
+
+As browsers are different, benchpress has plugins to normalizes these events.
+
+
+# Features
+
+* Provides a loop (so called "Sampler") that executes the benchmark multiple times
+* Automatically waits/detects until the browser is "warm"
+* Reporters provide a normalized way to store results:
+  - console reporter
+  - file reporter
+  - Google Big Query reporter (coming soon)
+* Supports micro benchmarks as well via `console.time()` / `console.timeEnd()`
+  - `console.time()` / `console.timeEnd()` mark the timeline in the DevTools, so it makes sense
+    to use them in micro benchmark to visualize and understand them, with or without benchpress.
+  - running micro benchmarks in benchpress leverages the already existing reporters,
+    the sampler and the auto warmup feature of benchpress.
+
+
+# Supported browsers
+
+* Chrome on all platforms
+* Mobile Safari (iOS)
+* Firefox (work in progress)
+
+
+# How to write a benchmark
+
+A benchmark in benchpress is made by an application under test
+and a benchmark driver. The application under test is the
+actual application consisting of html/css/js that should be tests.
+A benchmark driver is a webdriver test that interacts with the
+application under test.
+
+
+## A simple benchmark
+
+Let's assume we want to measure the script execution time, as well as the render time
+that it takes to fill a container element with a complex html string.
+
+The application under test could look like this:
+
+```
+index.html:
+
+<button id="reset" onclick="reset()">Reset</button>
+<button id="fill" onclick="fill()">fill innerHTML</button>
+<div id="container"></div>
+<script>
+  var container = document.getElementById('container');
+  var complexHtmlString = '...'; // TODO
+
+  function reset() { cotainer.innerHTML = ''; }
+
+  function fill() {
+    container.innerHTML = complexHtmlString;
+  }
+</script>
+```
+
+A benchmark driver could look like this:
+
+```
+// A runner contains the shared configuration
+// and can be shared across multiple tests.
+var runner = new Runner(...);
+
+driver.get('http://myserver/index.html');
+
+var resetBtn = driver.findElement(By.id('reset'));
+var fillBtn = driver.findElement(By.id('fill'));
+
+runner.sample({
+  id: 'fillElement',
+  // Prepare is optional...
+  prepare: () {
+    resetBtn.click();
+  },
+  execute: () {
+    fillBtn.click();
+    // Note: if fillBtn would use some asynchronous code,
+    // we would need to wait here for its end.
+  }
+});
+```
+
+## Measuring in the browser
+
+If the application under test would like to, it can measure on its own.
+E.g.
+
+```
+index.html:
+
+<button id="measure" onclick="measure()">Measure document.createElement</button>
+<script>
+  function measure() {
+    console.time('createElement*10000');
+    for (var i=0; i<100000; i++) {
+      document.createElement('div');
+    }
+    console.timeEnd('createElement*10000');
+  }
+</script>
+```
+
+When the `measure` button is clicked, it marks the timeline and creates 10000 elements.
+It uses the special names `createElement*10000` to tell benchpress that the
+time that was measured is for 10000 calls to createElement and that benchpress should
+take the average for it.
+
+A test driver for this would look like this:
+
+````
+driver.get('.../index.html');
+
+var measureBtn = driver.findElement(By.id('measure'));
+runner.sample({
+  id: 'createElement test',
+  microMetrics: {
+    'createElement': 'time to create an element (ms)'
+  },
+  execute: () {
+    measureBtn.click();
+  }
+});
+````
+
+When looking into the DevTools Timeline, we see a marker as well:
+![Marked Timeline](marked_timeline.png)
+
+# Best practices
+
+* Use normalized environments
+  - metrics that are dependent on the performance of the execution environment must be executed on a normalized machine
+  - e.g. a real mobile device whose cpu frequency is set to a fixed value
+  - e.g. a calibrated machine that does not run background jobs, has a fixed cpu frequency, ...
+
+* Use relative comparisons
+  - relative comparisons are less likely to change over time and help to interpret the results of benchmarks
+  - e.g. compare an example written using a ui framework against a hand coded example and track the ratio
+
+* Assert post-commit for commit ranges
+  - running benchmarks can take some time. Running them before every commit is usually too slow.
+  - when a regression is detected for a commit range, use bisection to find the problematic commit
+
+* Repeat benchmarks multiple times in a fresh window
+  - run the same benchmark multiple times in a fresh window and then take the minimal average value of each benchmark run
+
+* Use force gc with care
+  - forcing gc can skew the script execution time and gcTime numbers,
+    but might be needed to get stable gc time / gc amount numbers
+
+* Open a new window for every test
+  - browsers (e.g. chrome) might keep JIT statistics over page reloads and optimize pages differently depending on what has been loaded before
+
+# Detailed overview
+
+![Overview](overview.svg)
+
+Definitions:
+
+* valid sample: a sample that represents the world that should be measured in a good way.
+* complete sample: sample of all measure values collected so far
+
+Components:
+
+* Runner
+  - contains a default configuration
+  - creates a new injector for every sample call, via which all other components are created
+
+* Sampler
+  - gets data from the metrics
+  - reports measure values immediately to the reporters
+  - loops until the validator is able to extract a valid sample out of the complete sample (see below).
+  - reports the valid sample and the complete sample to the reporters
+
+* Metric
+  - gets measure values from the browser
+  - e.g. reads out performance logs, DOM values, JavaScript values
+
+* Validator
+  - extracts a valid sample out of the complete sample of all measure values.
+  - e.g. wait until there are 10 samples and take them as valid sample (would include warmup time)
+  - e.g. wait until the regression slope for the metric `scriptTime` through the last 10 measure values is >=0, i.e. the values for the `scriptTime` metric are no more decreasing
+
+* Reporter
+  - reports measure values, the valid sample and the complete sample to backends
+  - e.g. a reporter that prints to the console, a reporter that reports values into Google BigQuery, ...
+
+* WebDriverAdapter
+  - abstraction over the used web driver client
+  - one implementation for every webdriver client
+    E.g. one for selenium-webdriver Node.js module, dart async webdriver, dart sync webdriver, ...
+
+* WebDriverExtension
+  - implements additional methods that are standardized in the webdriver protocol using the WebDriverAdapter
+  - provides functionality like force gc, read out performance logs in a normalized format
+  - one implementation per browser, e.g. one for Chrome, one for mobile Safari, one for Firefox
+
+