Merge release-0_12 into master

Improve test coverage for Slice test

Unknown frame test

Add test coverage for gpr_refn() function

Deduplication of health.proto and math.proto

Use updated metadata auth call credentials core API in
Python (and migrate to Cython).

Node benchmark service

Fix string literals to hit the cases theyre supposed to

Fix windows build

add test for pluck after shutdown

Fix fuzzing detected crash

Fix typo

Make census_tag_set the only field of census_context

add test for global plugin

Fix memory leak

remove unused code

Improve test coverage for slice_buffer

Adding common tests for TSI.

Merge release-0.12 into master

Handle limitations on where dynamic table size updates can be placed