Use explicit String.concat in OTelDurationConverter

This is done because OTelDurationConverter is used at startup
and the use of "+" forces the use of StringConcatFactory
which is rather heavy and not beneficial in this simple case

Author: geoand

extensions/opentelemetry/runtime/src/main/java/io/quarkus/opentelemetry/runtime/OpenTelemetryRecorder.java

@@ -162,9 +162,9 @@ public String convert(final String value) throws IllegalArgumentException, NullP
             }
 
             try {
-                return duration.toMillis() + "ms";
+                return String.valueOf(duration.toMillis()).concat("ms");
             } catch (Exception ignored) {
-                return duration.toSeconds() + "s";
+                return String.valueOf(duration.toSeconds()).concat("s");
             }
         }
     }