Support for subsystem in grpc prometheus counter and histogram metrics

[rohsaini]

Changes

Introduction of apis: WithSubsystem() and WithHistogramSubsystem()

Verification

Tested locally with this new package of go-grpc-middleware

[johanbrandhorst]

Can you add some tests that exercises this option and checks that the output contains the subsystem?

[rohsaini]

Can you add some tests that exercises this option and checks that the output contains the subsystem?

added new test cases for client and server metrics both.

cd /home/ubuntu/datadisk/CLONES/go-grpc-middleware-subsystem/providers/prometheus && go test -v -race ./...
=== RUN TestClientInterceptorSuite
=== RUN TestClientInterceptorSuite/TestStartedStreamingIncrementsStarted
=== RUN TestClientInterceptorSuite/TestStreamingIncrementsMetrics
=== RUN TestClientInterceptorSuite/TestUnaryIncrementsMetrics
=== RUN TestClientInterceptorSuite/TestWithSubsystem
=== NAME TestClientInterceptorSuite
interceptor_suite.go:154: stopped grpc.Server at: 127.0.0.1:40133
--- PASS: TestClientInterceptorSuite (0.24s)
--- PASS: TestClientInterceptorSuite/TestStartedStreamingIncrementsStarted (0.00s)
--- PASS: TestClientInterceptorSuite/TestStreamingIncrementsMetrics (0.01s)
--- PASS: TestClientInterceptorSuite/TestUnaryIncrementsMetrics (0.00s)
--- PASS: TestClientInterceptorSuite/TestWithSubsystem (0.00s) <<<<<<<<
=== RUN TestServerInterceptorSuite
=== RUN TestServerInterceptorSuite/TestContextCancelledTreatedAsStatus
=== RUN TestServerInterceptorSuite/TestRegisterPresetsStuff
=== RUN TestServerInterceptorSuite/TestStartedStreamingIncrementsStarted
=== RUN TestServerInterceptorSuite/TestStreamingIncrementsMetrics
=== RUN TestServerInterceptorSuite/TestUnaryIncrementsMetrics
=== RUN TestServerInterceptorSuite/TestWithSubsystem
=== NAME TestServerInterceptorSuite
interceptor_suite.go:154: stopped grpc.Server at: 127.0.0.1:38755
--- PASS: TestServerInterceptorSuite (0.73s)
--- PASS: TestServerInterceptorSuite/TestContextCancelledTreatedAsStatus (0.10s)
--- PASS: TestServerInterceptorSuite/TestRegisterPresetsStuff (0.03s)
--- PASS: TestServerInterceptorSuite/TestStartedStreamingIncrementsStarted (0.20s)
--- PASS: TestServerInterceptorSuite/TestStreamingIncrementsMetrics (0.11s)
--- PASS: TestServerInterceptorSuite/TestUnaryIncrementsMetrics (0.00s)
--- PASS: TestServerInterceptorSuite/TestWithSubsystem (0.00s) <<<<<<<<
PASS
ok github.com/grpc-ecosystem/go-grpc-middleware/providers/prometheus 1.006s

[rohsaini]

Will raise it again. Closing due to some CLA issues.