grpc_test.go: Add test for using the NoDebugStack() option

[evanj]

Also add WithEnv and WithServiceVersion() options, since that
should more accurately represents how tracing and the interceptor is
typically configured. Example output now looks like:

goos: darwin
goarch: amd64
pkg: gopkg.in/DataDog/dd-trace-go.v1/contrib/google.golang.org/grpc
cpu: Intel(R) Core(TM) i7-8569U CPU @ 2.80GHz
BenchmarkUnaryServerInterceptor/ok_no_metadata-8         	  205440	      5295 ns/op	    5006 B/op	      53 allocs/op
BenchmarkUnaryServerInterceptor/ok_with_metadata_no_parent-8         	  189415	      6144 ns/op	    5635 B/op	      58 allocs/op
BenchmarkUnaryServerInterceptor/ok_with_metadata_with_parent-8       	  219744	      5451 ns/op	    4724 B/op	      49 allocs/op
BenchmarkUnaryServerInterceptor/ok_no_metadata_with_analytics_rate-8 	  214992	      5470 ns/op	    5214 B/op	      55 allocs/op
BenchmarkUnaryServerInterceptor/error_no_metadata-8                  	   91838	     12414 ns/op	   11397 B/op	      70 allocs/op
BenchmarkUnaryServerInterceptor/error_no_metadata_no_stack-8         	  169959	      6638 ns/op	    6312 B/op	      57 allocs/op

[evanj]

Oops I split this into #1160 which has still not yet been merged. That PR should be merged instead. Closing.