Address comments.

Author: visumickey

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

@@ -186,9 +186,6 @@ private void checkAttribute(@Nullable String key, @Nullable String value) {
       throw new IllegalArgumentException(
           "HttpMetric has been logged already so unable to modify attributes");
     }
-    if (key == null || value == null) {
-      throw new IllegalArgumentException("Attribute must not have null key or value.");
-    }
 
     if (!customAttributesMap.containsKey(key)
         && customAttributesMap.size() >= Constants.MAX_TRACE_CUSTOM_ATTRIBUTES) {

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

@@ -631,10 +631,6 @@ private void checkAttribute(@NonNull String key, @NonNull String value) {
           String.format(Locale.ENGLISH, "Trace '%s' has been stopped", name));
     }
 
-    if (key == null || value == null) {
-      throw new IllegalArgumentException("Attribute must not have null key or value.");
-    }
-
     if (!customAttributesMap.containsKey(key)
         && customAttributesMap.size() >= Constants.MAX_TRACE_CUSTOM_ATTRIBUTES) {
       throw new IllegalArgumentException(

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

@@ -28,8 +28,6 @@
 /** An abstract class providing an interface to validate PerfMetric */
 public abstract class PerfMetricValidator {
 
-  private String format;
-
   /**
    * Creates a list of PerfMetricValidator classes based on the contents of PerfMetric
    *
@@ -146,9 +144,9 @@ public static String validateAttribute(@NonNull Map.Entry<String, String> attrib
     String key = attribute.getKey();
     String value = attribute.getValue();
     if (key == null || key.length() == 0) {
-      return "Attribute key must not be null";
+      return "Attribute key must not be null or empty";
     } else if (value == null || value.length() == 0) {
-      return "Attribute value must not be null";
+      return "Attribute value must not be null or empty";
     } else if (key.length() > Constants.MAX_ATTRIBUTE_KEY_LENGTH) {
       return String.format(
           Locale.US,

firebase-perf/src/test/java/com/google/firebase/perf/metrics/validator/FirebasePerfTraceValidatorTest.java

@@ -194,7 +194,7 @@ public void traceValidator_invalidCustomAttriubute_skipsAttribute() {
     assertThat(new FirebasePerfTraceValidator(trace.build()).isValidPerfMetric()).isFalse();
 
     trace = trace.clone();
-    trace.clearCustomAttributes().putCustomAttributes("value", "");
+    trace.clearCustomAttributes().putCustomAttributes("", "value");
     assertThat(new FirebasePerfTraceValidator(trace.build()).isValidPerfMetric()).isFalse();
 
     StringBuilder longString = new StringBuilder();