Centralize metric name definitions
Description
This addresses #277 (closed) by moving all the metric names definitions to a single file.
(The actual values of those constants remains the same, so these changes are non-breaking.)
Changes proposed in this merge request:
- Add
buildgrid/server/metric_names.py
- Move the definitions in
buildgrid/server/metric_utils.py
and ``buildgrid/server/server.py` to that file - Slightly change naming of some constants to be consistent
Issues addressed
Closes #277 (closed)