Add link for Prometheus limitations to Metrics docs

Author: ckbedwell

src/data/markdown/translated-guides/en/02 Using k6/02 Metrics.md

@@ -109,7 +109,7 @@ For details of all metrics, refer to the [Metrics reference](/using-k6/metrics/r
 
 ## Metric name restrictions
 
-Metric names must comply with OpenTelemetry and Prometheus limitations.
+Metric names must comply with OpenTelemetry and [Prometheus limitations](https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels).
 
 That means they must not start with a number and a metric name can be 1 to 63 symbols of:
 1. any Unicode Letters