add 'explicit' restriction. (#12643)

Commit Message: The intent, when providing Stats::Utility::counterFromElements, is that dynamic segments should be easy to construct, but still searchable. We should be trying to avoid dynamic segments whenever possible, so having them implicitly created from string data is not idea.
Additional Description:
Risk Level: none for the repo, but possibly will require trivial edits outside the repo
Testing: //test/...
Docs Changes: n/a
Release Notes: n/a

Signed-off-by: Joshua Marantz <jmarantz@google.com>

source/common/stats/utility.h

@@ -27,7 +27,7 @@ class DynamicName : public absl::string_view {
   // This is intentionally left as an implicit conversion from string_view to
   // make call-sites easier to read, e.g.
   //    Utility::counterFromElements(*scope, {DynamicName("a"), DynamicName("b")});
-  DynamicName(absl::string_view str) : absl::string_view(str) {}
+  explicit DynamicName(absl::string_view str) : absl::string_view(str) {}
 };
 
 /**