Add more details of the http request

[dhananjaysathe]

Use nt/http/httptrace for getting more detailed metrics from the http probes

• Expose time taken for each phase dns resolution, connection, tls handshake, request send and reciept of first byte.
• use end misc latency times from net/http/httptrace callbacks directly
• Backward compatible with original http probe

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[dhananjaysathe]

cc @manugarg this adds more fine-grained metrics

[manugarg]

@dhananjaysathe Sorry for the delay in reviewing this PR. Unfortunately, adding more metrics affects everyone, so we'd want to enable them only through a configurable option. But, before you make any further changes, let me take a look at it.