Merge pull request #5013 from a-veitch/tag_set_to_context

BUILD

Makefile

binding.gyp

build.yaml

gRPC.podspec

grpc.def

grpc.gemspec

include/grpc/census.h

package.json

src/core/census/context.c

src/core/census/context.h

src/core/census/placeholders.c

src/core/census/tag_set.c

src/python/grpcio/grpc_core_dependencies.py

src/ruby/ext/grpc/rb_grpc_imports.generated.c

src/ruby/ext/grpc/rb_grpc_imports.generated.h

test/core/census/context_test.c

test/cpp/qps/client_async.cc

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json