all digest meters forward to percentile variant

This is a first step if phasing applications from digest to
histogram based percentile approximation. Both will be published
and we can then drop the digest variants after enough data is
available and dashboards have been updated.

spectator-reg-tdigest/src/main/java/com/netflix/spectator/tdigest/TDigestRegistry.java

@@ -17,6 +17,8 @@
 
 import com.netflix.spectator.api.*;
 import com.netflix.spectator.api.Counter;
+import com.netflix.spectator.api.histogram.PercentileDistributionSummary;
+import com.netflix.spectator.api.histogram.PercentileTimer;
 
 import javax.inject.Inject;
 import java.util.concurrent.TimeUnit;
@@ -41,11 +43,13 @@ public TDigestRegistry(Registry registry, TDigestConfig config) {
   }
 
   @Override protected TDigestDistributionSummary newDistributionSummary(Id id) {
-    return new TDigestDistributionSummary(newDigest(id), underlying.distributionSummary(id));
+    DistributionSummary summary = PercentileDistributionSummary.get(underlying, id);
+    return new TDigestDistributionSummary(newDigest(id), summary);
   }
 
   @Override protected TDigestTimer newTimer(Id id) {
-    return new TDigestTimer(newDigest(id), underlying.timer(id));
+    Timer timer = PercentileTimer.get(underlying, id);
+    return new TDigestTimer(newDigest(id), timer);
   }
 
   private StepDigest newDigest(Id id) {