Add query logging callbacks and context manager

[dcwatson]

Another take on query logging (#629), with a similar approach to existing listeners. Consider this a WIP - I'm happy to write documentation and more tests if this approach is preferable to #630.

[elprans]

Passing a con_ref is probably unnecessary?

[dcwatson]

I likely wouldn't use it, so happy to remove it, but I put it there so you could potentially log queries by host.

[elprans]

A concern here is potentially retaining references to free-d connections. Other callbacks take it, of course, but that's an API decision I've come to regret. Perhaps we can pass connection's addr and params instead?

[dcwatson]

Works for me!

[elprans]

I think a context manager would probably be nicer/reduce duplication.

[dcwatson]

Agreed, change incoming.

[elprans]

You could go further and pull self._log_query into the context manager and that way you'll be able to log query errors too, e.g.

Suggested change

	with utils.timer() as t:
	result, stmt = await self._do_execute(
	query,
	executor,
	timeout,
	record_class=record_class,
	ignore_custom_codec=ignore_custom_codec,
	)
	self._log_query(query, args, timeout, t.elapsed)
	with self._time_and_log(query, args, timeout):
	result, stmt = await self._do_execute(
	query,
	executor,
	timeout,
	record_class=record_class,
	ignore_custom_codec=ignore_custom_codec,
	)

[dcwatson]

Great idea, I feel silly for not considering error logging as part of this.

[elprans]

Suggested change

	['query', 'args', 'timeout', 'elapsed', 'exception', 'addr', 'params'])
	['query', 'args', 'timeout', 'elapsed', 'exception', 'conn_addr', 'conn_params'])