Metric Normalization note on w (#1219)

src/docs/sdk/metrics.mdx

@@ -63,11 +63,13 @@ class Aggregator:
 
 Normalization shall be done for `statsd` envelope items with unicode support if possible.
 
-* **Namespaces and Units:** regex `[^\w]+` with no replacement character.
-* **Metric Keys/Names:** regex `[^\w\-.]+` with replacement character `_`.
-* **Tag Keys:** regex `[^\w\-.\/]+` with no replacement character.
+* **Namespaces and Units:** regex `[^a-zA-Z0-9_]+` with no replacement character.
+* **Metric Keys/Names:** regex `[^a-zA-Z0-9_\-.]+` with replacement character `_`.
+* **Tag Keys:** regex `[^a-zA-Z0-9_\-.\/]+` with no replacement character.
 * **Tag Values:** See below
 
+{/* Internal note: We can't use `\w` instead of `a-zA-Z0-9` because on some Regex engines `\w` includes chars such as `ä`, `ö` or `é`. */}
+
 ### Tag Values Replacement Map
 
 Tag Values support the full UTF-8 character range, except for control characters.