release-23.2: streamingccl: replace replication lag metric with a replicated time metric

[blathers-crl]

Backport 1/1 commits from #115205 on behalf of @msbutler.

/cc @cockroachdb/release

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This patch removes the replication lag metric and dbconsole graph as it's wrong
if more than one replication stream is running, if the job coordinator moves,
or if the job is paused.

This patch adds the physical_replication.replicated_time_seconds metric which
tracks the replicated time as seconds since the unix epoch. The metric uses
seconds instead of nanos because the the time() prom function returns the
seconds since the unix epoch, allowing users to easily alert if
time()-physical_replication.replicated_time_seconds is too large.

Lastly, this patch removes 2 c2c metrics that seemed useless.

Informs #113951

Release note (ui change): this patch remotes the
physical_replication.frontier_lag_nanos metric and dbconsole graph is it's
sometimes wrong. This patch adds physical_replication.replicated_time_seconds
metric which users may use for alerting.

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Release justification: c2c metrics only change

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is