pkg/util/log: increment metric when failing to connect to fluentbit

Previous patches introduced the infrastructure necessary to record
metrics within the log package without introducing circular
dependencies.

With all that plumbing done, we can now increment the metric
representing fluentbit connection errors whenever a fluent-server
log sink fails to establish a connection.

Release note (ops change): This patch introduces the counter
metric `fluent.sink.conn.errors` to the CockroachDB tsdb,
which is exposed to `/_status/vars` clients as
`fluent_sink_conn_errors`. The metric is incremented whenever
a `fluent-server` log sink fails to establish a connection to
the log sink pointed to by the `address` for the sink in the
provided log config.

pkg/util/log/fluent_client.go

@@ -74,6 +74,7 @@ func (l *fluentSink) output(b []byte, opts sinkOutputOptions) (err error) {
 	}
 
 	if err := l.ensureConnLocked(b); err != nil {
+		logging.metrics.IncrementCounter(FluentSinkConnectionError, 1)
 		return err
 	}
 	return l.tryWriteLocked(b)