* add response body size metric to InstrumentedTransport, add InstrumentedTransport to RetryableHTTPClient * add doc to responseSizeCounterReadCloser, remove unused field from test * record response using ContentLength
* add response body size metric to InstrumentedTransport, add InstrumentedTransport to RetryableHTTPClient * add doc to responseSizeCounterReadCloser, remove unused field from test * record response using ContentLength