fix(database_observability.postgres): Use of defer in explain_plans collector

[cristiangreco]

Brief description of Pull Request

Fix explain_plans collector so that a cached query is finalised immediately instead of being deferred (which prevented correct update of processedCount and thus made batch-size limit ineffective).

Pull Request Details

Followup from #6804 (comment)

Issue(s) fixed by this Pull Request

n.a.

Notes to the Reviewer

PR Checklist

• Documentation added
• Tests updated
• Config converters updated
• This pull request was substantially generated with AI assistance (see the GenAI policy)

[Copilot]

Pull request overview

Fixes database_observability.postgres explain plan collection so the batch-size limit is enforced correctly by finalizing each cached query immediately (instead of deferring cache cleanup / accounting until the end of fetchExplainPlans).

Changes:

• Replace per-iteration defer cleanup with immediate per-query finalization and processedCount increment.
• Extract per-query processing logic into processExplainPlan and return a denylist decision to the caller.

[Copilot]

Pull request overview

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

Suppressed comments (2)

internal/component/database_observability/postgres/collector/explain_plans.go:626

• This error log uses c.logger, which drops the per-query context (query_id/datname) already attached to the local logger. Logging via logger helps operational debugging when Loki output emission fails.

	if err := c.sendExplainPlansOutput(
		qi.datname,
		qi.queryId,
		generatedAt,
		database_observability.ExplainProcessingResultSuccess,
		"",
		explainPlanOutput,
	); err != nil {
		c.logger.Error("failed to send explain plan output", "err", err)
	}

internal/component/database_observability/postgres/collector/explain_plans.go:538

• This log line uses c.logger even though a request-scoped logger with query_id context is already available. Using logger here makes the error easier to correlate to the specific query being processed.

This issue also appears on line 617 of the same file.

	if strings.HasSuffix(qi.queryText, "...") {
		err := c.sendExplainPlansOutput(
			qi.datname,
			qi.queryId,
			generatedAt,
			database_observability.ExplainProcessingResultSkipped,
			"query is truncated",
			nil,
		)
		if err != nil {
			c.logger.Error("failed to send truncated query skip explain plan output", "err", err)
		}