[Segment Replication] Calculate Average Time taken metric for Segment Replication events in a Replica shard

[Rishikesh1159]

Is your feature request related to a problem? Please describe.
With issue being resolved, we now have a _cat/segment_replication API to fetch metrics related to segment replication events on a replica shard.

We now want to add additional metric : Average Time taken. Which will be moving average of time taken by all segment replication events in a replica shard.

Scope:
-> Calculate and add additional average time taken metric to the existing _cat/segment_replication API.
-> Write some tests to verify working of this new metric.

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