[Prometheus] Try to find sweet spot for group_interval
[Prometheus] Try to find sweet spot for group_interval

BUG=skia:6283

Change-Id: Ie42422de6ab3d3787bb2aa00efb67ef00aa21bd0
Reviewed-on: https://skia-review.googlesource.com/8981
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot