Support for metrics/insight into dial attempts

[ianrose14]

Depending on the application, tuning MaxIdleConns can be difficult. And indication that this value may be too low would be frequent Dial attempts. Currently there is no way to instrument the client to record dial attempts.

[ianrose14]

Note that #86 would be one way to solve this because the pluggable DialTimeout impl could handle all metrics.

[dormando]

Note: need to see how this relates to #185