Excessive allocations in Firebase Trace.incrementMetric (#7984)

**Fixes:**
#1923

**Quick Summary:**
Current PR addresses an optimization at
PerfMetricValidator.validateTraceName(), it also gates the Metric
logging after this validation succeed within Trace.incrementMetric()
(allowing the log building if the flag is enabled) and optimizes this
String building process (using String template instead of
String.format).

Author: OOS93

firebase-perf/src/main/java/com/google/firebase/perf/metrics/Trace.java

@@ -348,8 +348,16 @@ public void incrementMetric(@NonNull String metricName, long incrementBy) {
     // thread-safer
     Counter counter = obtainOrCreateCounterByName(metricName.trim());
     counter.increment(incrementBy);
-    logger.debug(
-        "Incrementing metric '%s' to %d on trace '%s'", metricName, counter.getCount(), name);
+    if (logger.isLogcatEnabled()) {
+      logger.debug(
+          "Incrementing metric '"
+              + metricName
+              + "' to "
+              + counter.getCount()
+              + " on trace '"
+              + name
+              + "'");
+    }
   }
 
   /**

firebase-perf/src/main/java/com/google/firebase/perf/metrics/validator/PerfMetricValidator.java

@@ -33,6 +33,8 @@ public abstract class PerfMetricValidator {
   private static final Pattern ATTRIBUTE_KEY_PATTERN =
       Pattern.compile("^(?!(firebase_|google_|ga_))[A-Za-z][A-Za-z_0-9]*");
 
+  private static final Constants.CounterNames[] validCounterNames = Constants.CounterNames.values();
+
   /**
    * Creates a list of PerfMetricValidator classes based on the contents of PerfMetric
    *
@@ -126,7 +128,6 @@ public static String validateMetricName(@Nullable String str) {
       return String.format(
           Locale.US, "Metric name must not exceed %d characters", Constants.MAX_COUNTER_ID_LENGTH);
     } else if (str.startsWith("_")) {
-      Constants.CounterNames[] validCounterNames = Constants.CounterNames.values();
       for (Constants.CounterNames counterName : validCounterNames) {
         if (counterName.toString().equals(str)) {
           return null;