chore(doc): Document how to add a new metric

docs/add-metric.md

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+# Add a metric
+
+This document explains how to add a new metric in the exporter:
+
+Requirements:
+
+- Metric must be relevant to create alert or visualization (🚩 Don't collect metrics just for convenient visualization)
+- Metric names and labels must follow the [Prometheus best practices](https://prometheus.io/docs/practices/naming/)
+
+Steps:
+
+1. Define metric names and labels
+
+1. Implement it
+
+    1. Add a new field in `rdsCollector` structure in `internal/app/exporter/exporter.go`
+    1. Add metrics description in `NewCollector()` function in `internal/app/exporter/exporter.go`
+    1. Collect the metrics
+    1. Export the result in `Collect()` in `internal/app/exporter/exporter.go`
+
+1. Add tests
+
+1. Test metrics in an AWS environment
+
+1. Add metric in the `README.md`
+
+1. Commit with `feat(metric): Add <metric> <short description>` message to mark it a new feature in the release's changelog.
+
+1. Optional. Update Grafana dashboards to display the metric
+
+1. Optional. Add alert using the new metric in [DMF](https://github.com/qonto/database-monitoring-framework)