Add tracing, fix some transport bugs wrt buffer_pools

include/grpc/grpc.h

src/core/ext/transport/chttp2/transport/chttp2_transport.c

src/core/lib/iomgr/buffer_pool.c

src/core/lib/iomgr/buffer_pool.h

src/core/lib/iomgr/tcp_client_posix.c

src/core/lib/iomgr/tcp_posix.c

src/core/lib/iomgr/tcp_server_posix.c

...y/credentials/google_default/google_default_credentials.c

src/core/lib/security/credentials/jwt/jwt_verifier.c

...core/lib/security/credentials/oauth2/oauth2_credentials.c

src/core/lib/surface/init.c

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

test/core/end2end/tests/buffer_pool_server.c

test/core/util/mock_endpoint.c

test/core/util/passthru_endpoint.c

test/core/util/port_server_client.c