Bug fixes, tracing for bdp estimation

doc/environment_variables.md

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

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

src/core/ext/transport/chttp2/transport/internal.h

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

src/core/lib/surface/init.c

src/core/lib/transport/bdp_estimator.c

src/core/lib/transport/bdp_estimator.h

test/core/util/passthru_endpoint.c