Merge branch 'master' into stress_tests_metrics

Makefile

include/grpc++/security/credentials.h

include/grpc/grpc.h

src/core/client_config/subchannel.h

src/core/iomgr/pollset_windows.c

src/core/security/credentials.c

src/core/security/credentials.h

src/core/security/google_default_credentials.c

src/core/security/security_connector.c

src/core/surface/byte_buffer_reader.c

src/core/surface/call.c

src/core/transport/chttp2/hpack_encoder.c

src/core/transport/chttp2/hpack_encoder.h

src/core/transport/chttp2/hpack_parser.c

src/core/transport/chttp2/hpack_table.c

src/core/transport/chttp2/hpack_table.h

src/core/transport/chttp2/internal.h

src/core/transport/chttp2/parsing.c

src/core/transport/chttp2/writing.c

src/core/transport/chttp2_transport.c