roachtest/clusterstats: ignore PromQL info-level annotations

[miraradeva]

CollectPoint and CollectInterval treated any non-empty Prometheus
warnings slice as a fatal error. With Prometheus 2.53+, this slice
now includes informational annotations like

PromQL info: metric might not be a counter, name does not end in
_total/_sum/_count/_bucket: "sys_host_disk_write_bytes"

which is emitted for queries such as rate(sys_host_disk_write_bytes[1m])
and is purely a naming-convention nudge -- the query result is still
valid. After the recent bump to Prometheus 2.53.5 (#170676), this
broke admission-control/disk-bandwidth-limiter outright (it t.Fatals
on collection errors) and dropped bandwidth samples in admission-
control/{index,single-node-index}-backfill, which already log the
error and continue.

Extract a handlePromWarnings helper that classifies entries by the
"PromQL info:" wire-format prefix:

• "PromQL info: ..." entries are logged and dropped.
• "PromQL warning: ..." entries, and any unprefixed entries (e.g.
  legacy remote-read warnings, which predate the PromQL annotation
  system), continue to surface as the same error string as before.

Prometheus' annotations package defines exactly two sentinel error
types (PromQLInfo and PromQLWarning), so the prefix check is the
actual wire-format contract; the client_golang v1 API we use flattens
both into a single []string with no structured alternative.

Resolves: #170841
See also: #170790, #170793, #170843

Release note: None

[trunk-io]

😎 Merged successfully - details.

[cockroach-teamcity]

This change is 

[miraradeva]

@cockroachdb/test-eng I gave this a shot, but I'm happy to scrap this if you have a better fix in mind.

[srosenberg]

Thanks! It looks like there is an attempt to separate infos from warnings [1], but it hasn't landed as of yet. Meanwhile, the infos filter does the job.

[1] prometheus/client_golang#1963

[miraradeva]

/trunk merge

[pav-kv]

blathers backport all

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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Branch release-all doesn't exist, skipping

Backport to branch all failed. See errors above.

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[pav-kv]

blathers backport 26.2 26.1 25.4 25.2

[blathers-crl]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #170841: branch-release-25.2, branch-release-25.4, branch-release-26.1, branch-release-26.2.

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