[FLINK-39127][metrics] Truncate OTel attribute values via config

[Izeren]

What is the purpose of the change

Part 3 of FLIP-553. Large OpenTelemetry metric attribute values can be rejected by OTel collectors as oversized payloads. This PR adds two configuration options to the OpenTelemetry metric reporter to keep attribute values within safe bounds and to track post-truncation metric name collisions without unbounded heap growth.

Brief change log

• New option transform.attribute-value-length-limits.<attribute-name> (with * as the global key) truncates metric attribute values to the configured length. 0 drops the attribute, negative values disable the limit for that attribute, and the * global key applies to any attribute without an explicit entry.
• New option transform.collision-tracking-max-slots (default 50000) bounds the memory footprint of the post-truncation collision tracker via an access-ordered LRU. 0 disables collision tracking. Malformed or negative values WARN and fall back to the default — a typo should not tear down metric reporting at JobManager startup.
• New package-private MetricAttributeTransformer applies the limits and tracks collisions. Serialized via the existing reporter lock (@NotThreadSafe).
• OpenTelemetryMetricReporter.open() builds the transformer from reporter config; notifyOfAddedMetric applies it before registration.
• Options live in OpenTelemetryReporterOptions, which is shared with the trace and event reporters. Their descriptions explicitly state that these two options apply only to the metric reporter and are ignored by the trace and event reporters.
• Empty-string attribute values are preserved under any non-zero configured limit.

Verifying this change

This change added tests and can be verified as follows:

• Unit tests in MetricAttributeTransformerTest covering: empty attribute values preserved under non-zero limits, access-ordered LRU hot-slot survives eviction, bounded slot tracking under maxSlots * 4 distinct-slot load, malformed NumberFormatException path falls back to default, negative max-slots falls back to default, global vs per-attribute limit interaction, 0 drop semantics, and negative-limit disable semantics.
• Unit tests in OpenTelemetryMetricReporterTest covering the reporter-level wiring.
• Integration test OpenTelemetryMetricReporterITCase#testAttributeValueTruncation verifying truncated attribute values reach the exporter.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no (the new ConfigOptions are @PublicEvolving; the transformer class is package-private)
• The serializers: no
• The runtime per-record code paths (performance sensitive): no (metric reporter registration path only)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs (option descriptions regenerated into docs/layouts/shortcodes/generated/open_telemetry_reporter_configuration.html) and JavaDocs on MetricAttributeTransformer and the new ConfigOptions in OpenTelemetryReporterOptions.

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Generated-by: Claude Code (Opus 4.7)

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CI report:

• 74ad2be Azure: SUCCESS

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