feat(plugins): add otel correlation, custom_metadata allowlist, and column projection to BigQuery analytics

Add three additive, off-by-default observability controls to
BigQueryAgentAnalyticsPlugin:

- Span-level Cloud Trace correlation: opt in via `enable_otel_correlation`
  (default off) to capture the ambient OpenTelemetry span context into
  attributes.otel.{span_id,trace_id} when valid (a best-effort join key). The
  typed span_id/parent_span_id columns remain the plugin's internal execution
  tree; their schema descriptions are corrected to say so and to point
  consumers at attributes.otel.span_id for span-level joins. No plugin-owned
  span is created.
- custom_metadata_allowlist: capture allowlisted event.custom_metadata keys
  (exact keys, or explicit "prefix:*" patterns) into
  attributes.custom_metadata.* through the existing truncation + sensitive-key
  redaction pipeline. Truncation sets is_truncated; redaction does not.
- payload_column_denylist: project payload columns (content, content_parts,
  attributes, latency_ms) out of the table at write time. Applied schema-first
  so the table schema, Arrow schema, row dict, and views stay consistent;
  identity/correlation columns are protected and raise ValueError. Denying
  content_parts disables GCS offload, and denying attributes alongside a
  non-empty custom_metadata_allowlist is rejected.

Default behavior is unchanged when none of the three configs are set.

Co-authored-by: Haiyuan Cao <haiyuan@google.com>
PiperOrigin-RevId: 940629822

Author: haiyuan-eng-google