chore: add session_replay_events to lag monitoring

[fuziontech]

Problem

no visibility into lag on session_replay_events

Changes

scrape it for visability

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How did you test this code?

[benjackwhite]

Looks good - only thing I'm not sure about is how the count works. The replay_events is an aggregating merge tree and might not fit the format expected?

[pauldambra]

Ah, because we're hipsters session_replay_events doesn't have an _timestamp column 😊

[fuziontech]

Without a _timestamp field we can't track lag. If you try to use max_last_timestamp you currently get December 31, 2299, 11:43 PM which is not at all helpful for lag lol. We see into the future! 👀

If we can add _timestamp digested into a SimpleAggregateFunction(max, DateTime64(6, 'UTC')) field that would work for this though

[pauldambra]

requires #16994 so that we have _timestamp to track

[posthog-bot]

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