Doc: update doc of heartbeat metric

[SteveLauC]

What does this PR do

Updates the doc of the heartbeat field of RaftMetrics and RaftDataMetrics as we store the timestamp itself rather than the duration since the last acknowledged heartbeat/replication timestamp since #1202.

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[drmingdrmer]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @SteveLauC)