Chore: update GRPC to 1.12.3

[mayurkale22]

this is essential for Node 12 ci job. Splitting the change from #504 PR.

[codecov-io]

Codecov Report

Merging #535 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
+ Coverage   95.23%   95.29%   +0.05%     
==========================================
  Files         146      147       +1     
  Lines        9828     9873      +45     
  Branches      539      538       -1     
==========================================
+ Hits         9360     9408      +48     
+ Misses        468      465       -3

Impacted Files	Coverage Δ
src/grpc-stats/server-stats.ts	100% <0%> (ø)	⬆️
...c/zpages-frontend/controllers/tracez.controller.ts	100% <0%> (ø)	⬆️
...es-frontend/controllers/traceconfigz.controller.ts	100% <0%> (ø)	⬆️
src/grpc-stats/client-stats.ts	100% <0%> (ø)	⬆️
src/zpages-frontend/controllers/rpcz.controller.ts	100% <0%> (ø)	⬆️
src/grpc-stats/stats-common.ts	100% <0%> (ø)	⬆️
src/zpages-frontend/routes.ts	100% <0%> (ø)	⬆️
...c/zpages-frontend/controllers/statsz.controller.ts	100% <0%> (ø)	⬆️
src/grpc-stats/common-distributions.ts	100% <0%> (ø)
test/test-instana.ts	100% <0%> (+11.53%)	⬆️

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