Census stats recording with InProcessTransport

Message sizes are counted in MessageFramer and MessageDeframer, which InProcessTransport doesn't use. Right now we simply do not record metrics for InProcessTransport at all. This may turn out to be needed, so I use this issue to track this missing feature.

Start by reading MessageFramer and MessageDeframer to understand how message sizes are counted, then compare that path with InProcessTransport, which does not use them. Determine how Census metrics should be recorded for InProcessTransport and define the expected behavior before identifying suitable tests; the issue does not name existing test files or a specific entry point.

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