Problems with BatchingQueue

[richvdh]

#10017 introduced a BatchingQueue implementation. While reviewing #10035 I noticed a few oddities that could do with cleaning up:

• The params to __init__ could do with docstrings, especially name - what is it used for?
• The prometheus metrics (synapse_util_batching_queue_number_queued etc) don't appear to include the name label in their result.
• If the key is already being processed when _process_queue starts, then _process_queue immediately returns (https://github.com/matrix-org/synapse/blob/v1.35.0rc1/synapse/util/batching_queue.py#L118-L119). However, it will pass through the finally block which will remove the key from self._processing_keys. This looks incorrect to me.
• PreserveLoggingContext isn't free. consider moving it outside the loops?
• It would be nice to document the behaviour on exceptions.

[erikjohnston]

• If the key is already being processed when _process_queue starts, then _process_queue immediately returns (https://github.com/matrix-org/synapse/blob/v1.35.0rc1/synapse/util/batching_queue.py#L118-L119). However, it will pass through the finally block which will remove the key from self._processing_keys. This looks incorrect to me.

This is true, though in practice that condition could never be hit as we had already checked it (worth fixing though).