Sample and display time graph states by rounded numbers

[tahini]

Instead of sampling data in timeline graphs by arbitrary value that may change queries at different times, we should display at some fixed times, so that for a given pixel-time, it will always be the same state that is returned instead of some arbitrary one depending on sampling (no more disappearing states). This will allow to fetch data only for the missing times instead of the whole range as currently if half the range is already in memory, saving precious network bandwidth.

Also, when visualizing, the shown times should be rounded values instead of arbitrary ones, unless the user specifies otherwise (could be customizable by user)

This feature was implemented in lttng-scope (https://github.com/lttng/lttng-scope). Could be a good base to look at.

[ebugden]

Also, when visualizing, the shown times should be rounded values instead of arbitrary ones, unless the user specifies otherwise (could be customizable by user)

I would argue that label numbers should always be rounded to a useful scale (ex. if zoomed into microseconds then round to microseconds) and that the user should not have control over this.

I can't think of a use case where not rounding would be a practical advantage that makes it worth the extra UI noise. Unless control over label rounding is explicitly requested and justified with a valuable use case, I don't think it would be worth investing time to give users control over this.

[PatrickTasse]

The issue about rounded values for axis labels is fixed with https://github.com/theia-ide/timeline-chart/pull/172. The original issue about sampling data remains to be addressed.