[Proposal] Insrtrument client for providing a set of valuable metrics

[pfreixes]

I would like to work on a PR for adding some sort of instrumentalization to the client, but before I would prefer to have some feedback just for the sake of understanding how easy or difficult would be to move forward with this PR.

The client would be providing a new method - for example, metrics - which would provide a as many metrics as many hosts the client is using, each host would inform about the following metrics:

• Connections create that have been created until now per host
• Operations executed per host
• Connections closed until now per host
• Create Connection latency (avg, p50, p99, upper) per host

These are specific metrics that are hard to observe from the caller side unless you use a modified version of this package, like this one [1].

For other metrics, like the latencies per operation, the caller has always the chance of wrapping any call. The caller would have the chance of enriching his metrics by calling the metrics endpoint and sending them over to a Time Series database.

[1] https://github.com/orijtech/gomemcache