Add doc_url field to match its recent addition in pprof proto.

github.com/google/pprof/pull/888 added this field in pprof proto, this
change adds it to the OTel profiling proto to match the change.

Author: aalexand

opentelemetry/proto/profiles/v1experimental/pprofextended.proto

@@ -123,6 +123,14 @@ message Profile {
   // Index into the string table of the type of the preferred sample
   // value. If unset, clients should default to the last sample value.
   int64 default_sample_type = 14;
+  // Documentation link for this profile. The URL must be absolute,
+  // e.g., http://pprof.example.com/cpu-profile.html
+  //
+  // The URL may be missing if the profile was generated by older code or code
+  // that did not bother to supply a link.
+  int64 doc_url = 19;  // Index into string table.
+
+  reserved 20 to max; // NEXT ID
 }
 
 // Represents a mapping between Attribute Keys and Units.