identify metrics: track agent and protocol version

[sukunrt]

Track AgentVersion and ProtocolVersion and transport used per connection.

[marten-seemann]

How do you make sure that this doesn't become an attack vector? The inputs are controlled by the attacker? Ar you planning to introduce a whitelist?

[p-shahi]

Copying

tracking agent version is a difficult since it's a user controlled field, so an attacker can generate lots of agent versions which would lead to creation of a lot of prometheus time series on our end.
@marten-seemann has some ideas around a useful construct where we allow applications to specify a regex. However we can take that up after we finish the initial set of metrics.

#2103 (comment)

[p-shahi]

closing in favor #2205