Currently both ProfileResult and CollectorResult print the time field in a human readable string format
(e.g. "time": "55.20315000ms"). When trying to parse this back to a long value, for example to use in
the planned high level java rest client, we can lose precision because of conversion and rounding issues.
This change adds a new additional field (`time_in_nanos`) to the profile response to be able to get the
original time value in nanoseconds back.
The old `time` field is only printed when the `?`human=true` flag in the url is set. This follow the behaviour for
all other stats-related apis. Also the format of the `time` field is slightly changed. Instead of always formatting
the output as a 10-digit ms value, by using the `XContentBuilder#timeValueField()` method we now print
the largest time unit present is used (e.g. "s", "ms", "micros").
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