opentelemetry: Missing locality should be empty string

From gRFC A78:

> If no locality information is available, the label will be set to the
> empty string.

Author: ejona86

opentelemetry/src/main/java/io/grpc/opentelemetry/OpenTelemetryMetricsModule.java

@@ -213,7 +213,7 @@ void recordFinishedAttempt() {
       if (module.localityEnabled) {
         String savedLocality = locality;
         if (savedLocality == null) {
-          savedLocality = "unknown";
+          savedLocality = "";
         }
         builder.put(LOCALITY_KEY, savedLocality);
       }

opentelemetry/src/test/java/io/grpc/opentelemetry/OpenTelemetryMetricsModuleTest.java

@@ -974,7 +974,7 @@ public void clientLocalityMetrics_missing() {
 
     io.opentelemetry.api.common.Attributes clientAttributesWithLocality
         = clientAttributes.toBuilder()
-        .put(LOCALITY_KEY, "unknown")
+        .put(LOCALITY_KEY, "")
         .build();
 
     assertThat(openTelemetryTesting.getMetrics())