[client-v2] More connection runtime metrics

[chernser]

There is a need to monitor connections that are created by a client.

Here is list of wanted metrics:
Connection pool (scope)

• connection request time (how fast connection is returned from pool, may include creation time)
• connection creation time
• failed connection requests(count, ratio)

[Paultagoras]

So based on discussions:

• connection request time is an Apache-level metric, not something we should be touching in our client. It would involve extending internal methods, which is a nightmare waiting to happen
• Added
• This is better added at the application level, as each call can be wrapped in a try/catch and the cause can be selected to user preference. Otherwise it would be overly generic and basic at the client level.

[Paultagoras]

Closed by #2131