Fix regex values in configuration example (#19453)

The values istio_* and envoy_* make less sense in regex because it will match this (https://regex101.com/r/ehe8T9/1).
The best regex values would be  istio_.* and envoy_.*

Author: ddalexvea

istio/README.md

@@ -248,8 +248,8 @@ instances:
     metrics:
     - '.*'
     exclude_metrics:
-      - istio_*
-      - envoy_*
+      - istio_.*
+      - envoy_.*
 
 ```