Add unit to new failsafe histogram (#15537)

Co-authored-by: Trask Stalnaker <[email protected]>

Author: jaydeluca

instrumentation/failsafe-3.0/library/src/main/java/io/opentelemetry/instrumentation/failsafe/v3_0/FailsafeTelemetry.java

@@ -100,6 +100,7 @@ public <R> RetryPolicy<R> createRetryPolicy(RetryPolicy<R> delegate, String retr
         meter
             .histogramBuilder("failsafe.retry_policy.attempts")
             .setDescription("Number of attempts for each execution.")
+            .setUnit("{attempt}")
             .ofLongs()
             .setExplicitBucketBoundariesAdvice(Arrays.asList(1L, 2L, 3L, 5L))
             .build();