Add three Praefect metrics to docs

What does this MR do?

This MR updates the docs to include information about three Gitaly metrics: gitaly_praefect_replication_latency_bucket, gitaly_praefect_replication_delay_bucket and gitaly_praefect_node_latency_bucket added in 12.10. While these three Gitaly metrics pertain to Praefect, eventually this table would contain information about Gitaly metrics (whether or not the involve Praefect).

This is in response to 🎫 201543 (available to GitLab team members with access to ZenDesk) and gitaly#3536 (closed).

The metrics and their associated MRs:

Metric	Type	Since	Description	Labels
gitaly_praefect_replication_latency_bucket	Histogram	12.10 MR	The amount of time it takes for replication to complete once the replication job has started.
gitaly_praefect_replication_delay_bucket	Histogram	12.10 MR	A measure of how much time passes between when the replication job is created and when it is started.
gitaly_praefect_node_latency_bucket	Histogram	12.10 MR	Latency in Gitaly returning health check information to Praefect, indicates Praefect connection saturation.

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Does this MR meet the acceptance criteria?

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