Further simplify metrics tracker.

[davinchia]

What

Follow up from ##19814, where we introduced the StreamStats object to consolidate/simplify some of the stats memory objects.

In this PR, we extend the StreamStats object to also include the emitted records and bytes.

How

• Make StreamStats into a proper object. We cannot use a record as record fields are immutable. We need mutable fields to count.
• Consolidate the emitted records into StreamStats.
• Take the chance to move all the stats/metrics related classes into a book_keeping package to keep things clean.

Recommended reading order

1. AirbyteMessageTracker.java and StreamStats.java - for the main changes.
2. Everything else.

🚨 User Impact 🚨

Are there any breaking changes? What is the end result perceived by the user? If yes, please merge this PR with the 🚨🚨 emoji so changelog authors can further highlight this if needed.

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[davinchia]

PTAL @pmossman and @gosusnp

Parker, I realised I should have tagged you on the original PR - my bad. I was working on this as part of the progress bar work and simplified some of it as I found it getting complicated. One question I had was around memory usage. There is a fair amount of comments on intending keeping memory low e.g the stream indexes. Was that something we saw and we have to be aware of?

Jimmy I don't think we should simplify interfaces yet. I think consolidating the underlying implementation gets us most of the way there and sets us up to tweak the interfaces later.

[pmossman]

@davinchia I can look through the PR later today, but to answer your question about memory usage, this was my first project at Airbyte and I was following the recommended implementation from the issue: #3247

I don't think we were ever actively seeing memory issues, I think it was more a preemptive approach to make sure we never let the memory usage of our stats grow to a point where it'd be concerning

[pmossman]

Looks good!