Skip counter measurement when start listening while processing

[JamesNK]

Someone could start listening to counters after something we want to measure (e.g. a connection) has started. This is a problem for some counters:

• Counter tag enrichment feature is only added to features if counters are enabled. It would be unexpected for the feature not to be set, listening to start, and the counter to be recorded to.
• Up/down counters, such as the current connections, must be paired together. If the connection starts without a listener, the counter isn't increased. If a listening starts, we don't want to reduce the counter by 1 because it will go negative.

Hosting already worked. It calculates counters being enabled when a connection starts.

Kestrel needed a lot of work. This PR adds a metric context that figures out what counters are enabled when a connection starts and uses those values for the life of the connection.

It's tough to flow a metrics context throughout a connection. This PR uses an internal feature that Kestrel always sets.

[JamesNK]

@halter73 Addresses your comment here: #47758 (review)

SignalR metrics is still to do.

[amcasey]

I think the goal here is to make each operation either report metrics or not report metrics for its entire duration, is that right?

[JamesNK]

SignalR done.

[amcasey]

Thanks! This seems much less prone to user error.

[JamesNK]

@halter73 @Tratcher Could you take a look when you have some time? I'd like to double check the internal feature Kestrel adds to flow the metrics context - IConnectionMetricsContextFeature - is fine.