[feature] add metrics for worker counts, and worker queue sizes (#4387)

should help to debug https://codeberg.org/superseriousbusiness/gotosocial/issues/4309

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4387
Reviewed-by: Daenney <daenney@noreply.codeberg.org>
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

internal/workers/worker_fn.go

@@ -85,6 +85,11 @@ func (p *FnWorkerPool) Stop() {
 	p.workers = p.workers[:0]
 }
 
+// Len returns number of currently active workers.
+func (p *FnWorkerPool) Len() int {
+	return len(p.workers)
+}
+
 // FnWorker wraps a queue.SimpleQueue{} which
 // it feeds from to provide it with function
 // tasks to execute. It does so in a single