Move GetSpanFromServerContext() to public header.

[g-easy]

Move GetSpanFromServerContext() to public header.

[g-easy]

Currently, consumers of this code need to do:

#include <grpcpp/opencensus.h>
#include "src/cpp/ext/filters/census/grpc_plugin.h"

The first line looks okay, but the second line is a bit odd (e.g. no grpcpp/ prefix). Is the second one considered a public header?

[Vizerai]

LGTM

[Vizerai]

Nobody said anything against having grpc_plugin.h within src/cpp when I had the PR reviewed, so I assumed that was ok.

[grpc-testing]

****************************************************************

libgrpc.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]


****************************************************************

libgrpc++.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]

[grpc-testing]

[trickle] No significant performance differences

[vjpai]

Because of issues including the Abseil dependence, let me recommend that there should be an explicit check that we're actually building with bazel. Something like:

#ifndef GRPC_BAZEL_BUILD
#error OpenCensus for gRPC is only supported when building with bazel
#endif

[g-easy]

Done.

[grpc-testing]

[microbenchmarks] No significant performance differences

[g-easy]

No idea what this "mergeable" business is. I was expecting the followup commit to become a "fixup" in the git rebase sense.

[grpc-testing]

****************************************************************

libgrpc.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]


****************************************************************

libgrpc++.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]

[grpc-testing]

[trickle] No significant performance differences

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,952,998      Total (=)      1,952,998

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,664,799      Total (>)     10,664,797

 No significant differences in binary sizes

[grpc-testing]

[microbenchmarks] No significant performance differences