Add alerts for availability issues and minor improvements to control cardinality

[pree-dew]

Follow up from issue https://github.com/modelcontextprotocol/registry/issues/509

1. Add alerts for critical failure cases.
2. Check possibilities of reducing number of buckets for latency.
3. Change dot from metric name to underscore.

[pree-dew]

@rdimitrov Please add if I have missed anything.

[rdimitrov]

I think that's good 🙏