[processor/tailsampling] Add feature gate to change default error_mode to ignore

[singhvishalkr]

I noticed the tailsampling processor's ottl_condition policy type defaults to error_mode: propagate, which drops the entire payload when any OTTL condition encounters an error. This can be unexpected - a single malformed record causes data loss.

This PR adds a feature gate processor.tailsamplingprocessor.defaultErrorModeIgnore that, when enabled, changes the default to ignore. With this setting, records that fail OTTL evaluation are skipped but other valid records are preserved. I modeled this on the same feature gates I just added for the routing and signaltometrics connectors (#48418, #48419).

The feature gate starts at alpha stage. Users can opt-in via --feature-gates=processor.tailsamplingprocessor.defaultErrorModeIgnore. Once stability is proven, the gate can progress to beta/stable to make ignore the permanent default.

Changes:

• Added feature gate definition in metadata.yaml
• Updated processor.go to use the feature-gated default when error_mode is not explicitly set
• Added docs to OTTLConditionCfg.ErrorMode explaining the feature gate
• Added unit test covering both gate states
• Created changelog entry

Fixes #48420

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