chore(datastore): Remove public visibility for MetricsRecorder (#12043)

Remove public visibility prior to release. Allow for modifications
before release.

Author: lqiu96

java-datastore/google-cloud-datastore/src/main/java/com/google/cloud/datastore/telemetry/MetricsRecorder.java

@@ -23,7 +23,7 @@
 import javax.annotation.Nonnull;
 
 /** Interface to record specific metric operations. */
-public interface MetricsRecorder {
+interface MetricsRecorder {
   /** Records the total latency of a transaction in milliseconds. */
   void recordTransactionLatency(double latencyMs, Map<String, String> attributes);