`$_path` is used by documentation tests to ignore a value from a
response, for example:
```
[source,js]
----
{
"count": 1,
"datafeeds": [
{
"datafeed_id": "datafeed-total-requests",
"state": "started",
"node": {
...
"attributes": {
"ml.machine_memory": "17179869184",
"ml.max_open_jobs": "20",
"ml.enabled": "true"
}
},
"assignment_explanation": ""
}
]
}
----
// TESTRESPONSE[s/"17179869184"/$body.$_path/]
```
That example shows `17179869184` in the compiled docs but when it runs
the tests generated by that doc it ignores `17179869184` and asserts
instead that there is a value in that field. This is required because we
can't predict things like "how many milliseconds will this take?" and
"how much memory will this take?".
Before this change it was impossible to use `$_path` when any component
of the path contained a `.`. This fixes the `$_path` evaluator to
properly escape `.`.
Closes#28770
a813492fe3