Add database plugin metrics around connections
This is a replacement for #15923 that takes into account recent lock
cleanup.
I went ahead and added back in the hanging plugin test, which I meant to
add in #15944 but forgot.
I tested this by spinning up a statsd sink in the tests and verifying I
got a stream of metrics:
```
$ nc -u -l 8125 | grep backend
test.swenson-Q9Q0L72D39.secrets.database.backend.connections.count.pgx.5.:1.000000|g
test.swenson-Q9Q0L72D39.secrets.database.backend.connections.count.pgx.5.:0.000000|g
test.swenson-Q9Q0L72D39.secrets.database.backend.connections.count.pgx.5.:1.000000|g
test.swenson-Q9Q0L72D39.secrets.database.backend.connections.count.pgx.5.:0.000000|g
```
We have to rework the shared gauge code to work without a full
`ClusterMetricSink`, since we don't have access to the core metrics from
within a plugin.
This only reports metrics every 10 minutes by default, but it solves
some problems we would have had with the gauge values becoming stale and
needing to be re-sent.
Co-authored-by: Tom Proctor <tomhjp@users.noreply.github.com>
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