* Add RecordN method to permit recording of N measurements simultaneously.
In serving we have many places where we try to record several metrics at the same time.
Besides very repetetive code, this also brings inefficiences, since metrics code has to do _some_ work
for every call, which is completely avoidable.
This way we can record N measurements with one hit.
* tests and nits
* review items per discussion with Evan:
- remove ros
- make measurements ...
* review
* Add metrics to webhook package
Add metricstest package for shared helper functions for testing metrics
* Address PR
* Cleanup
* Fix import paths to fix build issues
* Fix import package path for test file
* Remove unnecessary formatting from error message
* Remove helper function only used once
* Add metric name to all error messages, make checkRowTags testing helper function
* Add common histogram bucket generator function to metrics package
* Fix CheckStatsNotReported check
* Reset metrics before each test so the tests are idempotent
* Make CheckStatsNotReported conditional clearer
* split and update
* bug fixed
* change the copyright year
* use fake metadata in tests
* exclude test coverage for gcp_metadata
* add record
* add tests
* revert 2019 and add a new var
* change documents
* address comment
* change line
Victor Agababov
Matt Moore
Evan Anderson
Yanwei Guo
Matt Moore
Ignacio Cano
Annie Fu