db: add custom profile for open iterators

[jbowens]

Open iterators pin memtables and sstables. Iterators must not be kept open for a long duration to avoid bloating memory or disk space usage. Go supports custom pprof profiles, which we could leverage here. We would pprof.Profile.Add when an iterator is opened, and pprof.Profile.Remove when an iterator is closed, using the iterator pointer itself as the value argument.

Tangentially related to #723, another custom profile issue.

[nicktrav]

Building on top of this profiling mechanism, we could also keep track of when iters were created (and where), and provide a means of introspecting "long lived iters".

If we wire that up into Cockroach, I can imagine something like a debug page that prints out all the open iters sorted in descending order of age, along with where they were allocated, and possibly all the other iter stats (bytes scanned, skipped, etc.).

[nicktrav]

Linking #1609, cockroachdb/cockroach#68303 and cockroachlabs/support#1403 (internal), as they all would have benefited from the additional observability proposed here.

[jbowens]

Linking cockroachdb/cockroach#71481.

[nicktrav]

Linking cockroachdb/cockroach#79879.

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