fix(metrics): reset gauge vectors on recalculation to clear stale metrics

[Neha-Nimsatkar]

fix(metrics): reset gauge vectors on recalculation to clear stale metrics

In backend/pkg/metrics/metrics.go, appInstancePerChannelGaugeMetric and failedUpdatesGaugeMetric are updated via WithLabelValues(...).Set(...) whenever calculateMetrics() executes.

When an instance upgrades, gets deregistered, or an application/version combination is no longer returned by the database queries (GetAppInstancesPerChannelMetrics or GetFailedUpdatesMetrics), the Prometheus collector retains the previous label values indefinitely in memory.

This PR adds .Reset() calls on both GaugeVec collectors before setting fresh values so that stale time-series label combinations are properly cleared on each recalculation cycle.

How to use

Reviewers can inspect calculateMetrics() in backend/pkg/metrics/metrics.go to verify that appInstancePerChannelGaugeMetric.Reset() and failedUpdatesGaugeMetric.Reset() are invoked prior to metric iteration.

Testing done

• Ran go fmt ./pkg/metrics/... to ensure standard Go formatting.
• Verified compilation and static analysis using:

cd backend
go build ./pkg/metrics/...