Switch from std::map to absl::btree_map in MultivariateTimeSeries (#4348)
Switch from std::map to absl::btree_map in MultivariateTimeSeries (#4348)

* Switch from std::map to absl::btree_map in MultivariateTimeSeries

This is a microoptimization for the time range lookup.
Going over the elements within a range, `GetEntriesAffectedByTimeRange`
increments an iterator. absl::btree_map handles iterator increment much
faster than std::map.

This saves a few ms on a 5min capture.
Before: 80ms (http://screenshot/58kZywHWRs2458v)
After: 74ms (http://screenshot/9akLupbLcZNUKUB)

Bug: http://b/254449210
Test: Existing unit tests

* Fixup: format

Co-authored-by: Nikita Putikhin <0b9fe304d866d21fb22538a615661e29be263a64@google.com>
google/orbit,google/orbit,google/orbit,google/orbit