Quorum: Add ws and other metrics

[danimhr]

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[ali-behjati]

I'm generally a bit on the fence with the metrics library because it's very flexible and if a metric is used in two places, there's chances of error (typo/...) but i can tolerate it.

Apart from these metrics, i like to see these two metrics too:

• number of active subscribers (so we can see if there's a decrease)
• number of observations sent per signer (so we can detect if a signer goes down)

[danimhr]

I'm generally a bit on the fence with the metrics library because it's very flexible and if a metric is used in two places, there's chances of error (typo/...) but i can tolerate it.

Apart from these metrics, i like to see these two metrics too:

• number of active subscribers (so we can see if there's a decrease)
• number of observations sent per signer (so we can detect if a signer goes down)

I agree with your points. I chose to use the metrics crate because it's compatible with the Axum Prometheus. Also, the Quorum project is small, I think the current setup is fine for now.
We can define metric names as enum as well, if you think having it is necessary. Let me know if we have to add it before merge.

We can caculate the number of active subscribers by looking at the total number of ping updates sent by the server, which is why I didn’t add any other metric for active subscribers.

I’ll add the "observations per signer" metric in the upcoming commits.