Document metric name restrictions (#1205)

Co-authored-by: Oleg Bespalov <[email protected]>

Author: mstoykov

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

@@ -106,3 +106,18 @@ default ✓ [======================================] 1 VUs  00m03.8s/10m0s  1/1
 In that output, all the metrics that start with `http`, `iteration`, and `vu` are _built-in_ metrics, which get written to stdout at the end of a test.
 For details of all metrics, refer to the [Metrics reference](/using-k6/metrics/reference/).
 
+
+### Metric name restrictions
+
+<Blockquote mod="warning" title="">
+
+In the future, metrics names will be further limited to comply with OpenTelemetry and Prometheus limitations.
+
+That means metric names can be 1 to 63 symbols of ASCII letters, numbers, or  `_`. Metric names must not start with a number.
+
+</Blockquote>
+
+At the moment, a metric name can be 1 to 128 symbols of:
+1. any Unicode Letters
+2. any Unicode Number
+3. `.`, `_`, ` `, `!`, `?`. `/`, `#`, `(`. `)`, `<`, `>`, `%` and `-`