simplify per-topic metric collection

[cskiraly]

removing some of the repeated metrics code for per-topic metrics in PubSub and co.

[Menduist]

Discussion on discord: Tanguy — Today at 1:01 PM My only issue with the current technique is that it only works for inc (and to be fair, that's all we have right now)

Maybe have a proc metricTopicLabel(p: PubSub, t: string): string = p.knownTopics.containsOr(topic, "generic")

and then libp2p_pubsub_broadcast_subscriptions.inc(npeers, labelValues = p.metricTopicLabel(topic)) ? cskiraly — Today at 2:15 PM sounds like a good compromise. Tanguy — Today at 2:18 PM Btw, we could also remove the knownTopics from PubSub completely, it's just a copy from a constant in pubsub.nim That may simplify the code a little bit (don't need the PubSub in metricTopicLabel)

[dryajov]

very nice, much needed cleanup.