eth/protocols, prp/tracker: add support for req/rep rtt tracking

[karalabe]

This PR is a counterpart to #22581, adding metrics to the request/reply RTTs too.

Given that we (currently) do not have a centralized dispatcher which matches requests with replies, this PR introduces a global request tracker (per protocol). The tracker can be notified of outbound requests and inbound replies, and it will match them up and update the relevant metrics.

Since the request/reply pattern is stateful, there are a few caveats in the tracker:

• The tracker maintains a list of all currently pending requests. The only way to remove an item from this list is to either deliver a response; or for the request to time out. The timeout however is not the same as the downloader might use, rather a global number per protocol (5 min for eth, 1 min for snap). This allows us to a) detect loooong deliveries even if the calling code timed out and discarded the request; b) track expiries with a doubly linked list, making deletions and expiry O(1).
• Given that we track every send request, these can accumulate quite a bit if we have a huge peer count. In theory it shouldn't go over the number of peers / packet type, but it can temporarily on sync restarts or peer churn. I've added a metric to display these too to avoid surprises, but we might consider some hard enforced limit to make sure we can't go OOM due to some bug.

Open questions:

• I've used one singleton global tracker per protocol. Creating a scoped tracker in package eth (backend) and propagating it to all relevant call sites would become quite nasty; and our metrics can't differentiate between 2 node instances running within the same process either way. May or may not be a goo idea, wanted to emphasize it.
• We could fairly easily add the possibility to track request/reply sizes too. Unsure if this would be more help or more noise.

TL;DR

[holiman]

LGTM, if you add some kind of hard limit to the number of tracked requests (it can be large though)

[karalabe]

@holiman PTAL

Added a cap, looks like this when hit (I set it to 1 to test it):