fix: prevent npe in metrics during a batching retry (#4516)

* fix: prevent npe in metrics during a batching retry

Change-Id: I14d2fd195568fc9d827ec7ae1044d031a1d5cc04

* comment

Change-Id: I2d5876db67c2d8710d6930e4c64095113313cd34

Author: igorbernstein2

bigtable-client-core-parent/bigtable-hbase/src/main/java/com/google/cloud/bigtable/hbase/wrappers/veneer/metrics/MetricsApiTracerAdapter.java

@@ -156,9 +156,11 @@ public void attemptFailed(Throwable error, Duration delay) {
     lastRetryStatus = RetryStatus.ATTEMPT_RETRYABLE_FAILURE;
     rpcMetrics.markRetry();
     activeRpcCounter.dec();
-    BigtableClientMetrics.meter(
-            MetricLevel.Info, "grpc.errors." + Status.fromThrowable(error).getCode())
-        .mark();
+
+    // For batching, retries can be invoked despite the rpc status being OK because an entry failed.
+    Status status = (error == null) ? Status.OK : Status.fromThrowable(error);
+
+    BigtableClientMetrics.meter(MetricLevel.Info, "grpc.errors." + status.getCode()).mark();
   }
 
   @Override