fix: drop invalid OpenMetrics 2.0 exemplars instead of failing exposition

[dashpole]

Small follow-up to #894.

Per the OpenMetrics 2.0 specification failure modes, failures specific to exemplars should not cause the entire exposition to fail. Invalid exemplars (e.g. invalid timestamps, malformed labels) are now dropped so that the rest of the metric exposition succeeds.

"This specification advocates for transactional processing: any encoding, decoding, or validation errors must reject the whole MetricSet ingestion. A failed scrape is better than an inaccurate scrape or a partial metric view that breaks transactionality (e.g., scraping a portion of a StateSet MetricGroup, or scraping only one Counter out of two that are aggregated in a single alert expression).

There's one exception to this rule: failures specific to exemplars should not cause the entire exposition to fail. If an exemplar is malformed or invalid, it should be dropped or ignored, allowing the valid metric data to be ingested."

This also consolidates exemplar timestamp validation in validateExemplar20.

[Copilot]

Pull request overview

Updates OpenMetrics 2.0 exposition handling to drop invalid exemplars while preserving valid metric output.

Changes:

• Centralizes exemplar timestamp validation.
• Drops exemplars with invalid timestamps or labels.
• Adds regression coverage for invalid exemplars.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
expfmt/openmetrics_2_0_create.go	Implements exemplar validation and dropping behavior.
expfmt/openmetrics_2_0_create_test.go	Tests invalid exemplar handling.

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