Add metric for skipped certs per client

[goliel]

Hi,
We have migrated our certstream server from calidog's to this one and have been enjoying the added logs and metrics.
We had an issue with our consumer being too slow and certs being skipped, we have since fixed this issue but we would like to monitor it.

Right now we only monitor at the log level, Adding metric for the skipped amount would help us follow this issue better.

[d-Rickyy-b]

Since I totally missed commenting on that one: I really liked the idea so I implemented it yesterday and released v1.6.0. Thanks for your suggestion. Please tell me if the implementation works for you or if I missed something.

The name of the client is the IP:Port and it's used as a label for the certstreamservergo_skipped_certs metric. As soon as a client disconnects, the client's metric will be removed.
Using the IP:Port as client also means that as soon as a client reconnects, it gets a different label.