Add dropwizard contrib (#1423)

* Add dropwizard metrics producer

* Use Summary Type

* Fix reviews

* Fix reviews
diff --git a/build.gradle b/build.gradle
index 9ecd7b9..abf2413 100644
--- a/build.gradle
+++ b/build.gradle
@@ -168,6 +168,7 @@
         zipkinReporterVersion = '2.3.2'
         jaegerReporterVersion = '0.27.0'
         opencensusProtoVersion = '0.0.2'
+        dropwizardVersion = '3.1.2'
 
         libraries = [
                 appengine_api: "com.google.appengine:appengine-api-1.0-sdk:${appengineVersion}",
@@ -208,6 +209,7 @@
                 mockito: 'org.mockito:mockito-core:1.9.5',
                 spring_test: "org.springframework:spring-test:${springVersion}",
                 truth: 'com.google.truth:truth:0.30',
+                dropwizard: "io.dropwizard.metrics:metrics-core:${dropwizardVersion}",
         ]
     }
 
@@ -416,6 +418,7 @@
         name in ['opencensus-api',
                  'opencensus-contrib-agent',
                  'opencensus-contrib-appengine-standard-util',
+                 'opencensus-contrib-dropwizard',
                  'opencensus-contrib-exemplar-util',
                  'opencensus-contrib-grpc-metrics',
                  'opencensus-contrib-grpc-util',