Pin to Prometheus 0.15.+ for 1.9.x branch

We do not want to pick up new minor versions in a patch release.

Author: shakuzen

dependencies.gradle

@@ -35,8 +35,8 @@ def VERSIONS = [
         'io.grpc:grpc-stubs:latest.release',
         'io.grpc:grpc-alts:latest.release',
         'info.ganglia.gmetric4j:gmetric4j:latest.release',
-        'io.prometheus:simpleclient_common:latest.release',
-        'io.prometheus:simpleclient_pushgateway:latest.release',
+        'io.prometheus:simpleclient_common:0.15.+',
+        'io.prometheus:simpleclient_pushgateway:0.15.+',
         'javax.cache:cache-api:latest.release',
         'javax.inject:javax.inject:1',
         'javax.xml.bind:jaxb-api:2.3.+',