release-25.2: allstacks: improve buffer alloc sizing and adopt more widely

[tbg]

Backport 2/2 commits from #159279 on behalf of @tbg.

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In an escalation¹, we observed that on larger clusters,
multiple rounds of runtime.Stacks are required until the buffer
sizing algorithm as produced a large enough buffer.
Since grabbing stacks will cause stop-the-world pauses,
this is best avoided.

NB: *serverpb.StacksRequest is already populated using allstacks.Get.

Fixes #159285.

Release note (bug fix): the performance impact of the automatic goroutine dumper has been reduced by reducing the number of stop-the-world events it causes (especially on large, busy systems).

Epic: none

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Release justification:

Footnotes

1. https://github.com/cockroachlabs/support/issues/3495#issuecomment-3622936905 ↩

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

❌ PR #159392 does not comply with backport policy

Confidence: high
Explanation: The PR does not clearly fulfill any specific critical bug criteria under the backport policy. While it addresses performance improvements by reducing stop-the-world events, the description lacks evidence that this qualifies as a 'significant performance regression' or 'stability issue'. Furthermore, no feature flags are mentioned or introduced in this PR, making it non-compliant according to the backport policy. The PR does not mention removing any version gates, suggesting that it may be backward compatible. However, the 'Release justification' section in the PR body is incomplete, providing no explicit reasoning to support policy exception.
Recommendation: Reconsider the backport or provide a clearer justification and demonstration that the performance impact aligns with the critical bug criteria, or alternatively, gate the changes behind a feature flag.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is