DNS, SSL, and TCP metrics for Apache HttpClient

[shakuzen]

Add metrics for:

1. DNS resolution time (inject custom DnsResolver, delegating to the user-provided resolver, and timing).
2. SSL handshake time (inject SSLConnectionSocketFactory, delegating).
3. TCP connect time (also in custom SSLConnectionSocketFactory).

Apply these on custom PoolingHttpClientConnectionManager.

Feedback suggests that DNS resolution/SSL handshake can go up dramatically in noisy neighbor situations.

Originally posted by @jkschneider in #533 (comment)

[sahilkamboj334]

any update on this imlementaion @shakuzen ? are we planning these ?

[shakuzen]

No update from my side. A pull request may help get this done sooner. See apache/httpcomponents-client#555, which looks like it would be a useful new feature to help with implementing this. We aren't aware of much specific interest in this from our users, so it has not been prioritized. I've added the help wanted label.