Add back that a metric name cannot start with a number to Using metrics page

Author: ckbedwell

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

@@ -111,7 +111,7 @@ For details of all metrics, refer to the [Metrics reference](/using-k6/metrics/r
 
 Metric names must comply with OpenTelemetry and Prometheus limitations.
 
-That means a metric name can be 1 to 63 symbols of:
+That means they must not start with a number and a metric name can be 1 to 63 symbols of:
 1. any Unicode Letters
 2. any Unicode Number
 3. `_` (an underscore)