[Enhancement] Support suppressing minimal-value metrics to reduce export payload

Before Creating the Enhancement Request

• I have confirmed that this should be classified as an enhancement rather than a bug/feature.

Summary

This PR introduces an opt-in broker config suppressMinValueMetrics (default false, backward compatible) that suppresses exporting data points whose value is not greater than a minimal threshold.

Motivation

The consumer lag gauges are asynchronous ObservableGauges that emit one data point per consumer_group × topic × is_retry combination. In deployments with many consumer groups and topics, the vast majority of these combinations are idle at any given sampling instant, so their values are 0 — yet they are still recorded and exported on every collection cycle. This inflates both metric cardinality and the OTLP export payload.

In production, we observed zero-valued points accounting for ~95% of gauge data points and the bulk of the payload. Suppressing them is safe as long as the monitoring pipeline tolerates discontinuous series.

Describe the Solution You'd Like

• BrokerConfig: add suppressMinValueMetrics (default false).
• BrokerMetricsManager: add a general guard shouldRecordValue(currentValue, minValue). When suppression is enabled, a data point is recorded only if currentValue > minValue.
• Apply the guard at the record site of the 5 consumer lag gauges, all passing minValue = 0 for now, i.e. zero-valued points are dropped.

Describe Alternatives You've Considered

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Additional Context

When enabled, threshold-based dashboards/alerts (e.g. lag > N) are unaffected; the only difference is that idle series appear as gaps instead of a flat zero line. Therefore, before turning this on, make sure there are no alert rules based on absent() / absent_over_time() (or any equivalent "no data" detection) targeting these metrics.