fix metric description

Change-Id: I38ed23e8d31591fc2bd8d0e5cc1f15dd7c518bce

Author: igorbernstein2

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/metrics/BuiltinMetricsTracerFactory.java

@@ -87,7 +87,7 @@ public static BuiltinMetricsTracerFactory create(
     attemptLatencies2Histogram =
         meter
             .histogramBuilder(ATTEMPT_LATENCIES2_NAME)
-            .setDescription("Client observed latency per RPC attempt.")
+            .setDescription("Client observed latency per RPC attempt with transport labels.")
             .setUnit(MILLISECOND)
             .build();
     serverLatenciesHistogram =