[Metricbeat][Redis] Do not drop keyspace metrics when INFO keyspace reports extra fields

[aglowienka]

Proposed commit message

parseKeyspaceStats (metricbeat/module/redis/keyspace/data.go) only mapped a database line when
INFO keyspace split into exactly 3 or 4 comma-separated fields. Redis 7.4 added subexpiry and
Valkey 9.0 added keys_with_volatile_items; any further field makes the guard fail. When it fails
the line is silently skipped — no event, no error logged — so redis.keyspace stops producing data
entirely while redis.info keeps working.

This changes the guard to accept any line with at least 3 fields (>= 3). The parser is name-based
and the schema only extracts the fields it knows, so extra/unknown keyspace fields are ignored
safely and won't silently break collection again the next time a field is added.

Relates to #52178.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation (no user-facing fields change)
• [ ] I have made corresponding change to the default configuration files (n/a)
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Disruptive User Impact

None. The only behavior change is that keyspace lines carrying extra fields are no longer dropped.

How to test this PR locally

go test ./metricbeat/module/redis/keyspace/...

TestParseKeyspaceStats now includes a db line with extra fields and asserts it is still parsed.

Backport

The same bug affects the 8.19.x and 9.0–9.3 lines (see #52178), where redis.keyspace is
lost entirely on Valkey ≥ 9.0 / Redis ≥ 7.4. #47971 relaxed the guard to == 3 || == 4 on main
(9.4+) but was not backported; this change supersedes it with the more robust >= 3. Please
backport to 8.19 (and 9.0–9.3 if still applicable).

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @aglowienka? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
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To fixup this pull request, you need to add the backport labels for the needed
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• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[aglowienka]

CLA signed.
This bugfix should be backported to all active branches
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