Strengthen metric name structure according to Prometheus guidance

include/ConflictSet.h

@@ -91,7 +91,7 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
   /** Experimental! */
   struct MetricsV1 {
     /** A null-terminated string with static lifetime. Identifies this metric.
-     * Matches the regex [a-zA-Z_:][a-zA-Z0-9_:]*
+     * Matches the regex [a-zA-Z][a-zA-Z0-9_]*
      */
     const char *name;
     /** A null-terminated string with static lifetime. Describes this metric.