Allow passing already configured redis client to the storage adapter

src/Prometheus/Storage/Redis.php

@@ -40,7 +40,10 @@ public function __construct(array $options = array())
         }
 
         $this->options = array_merge(self::$defaultOptions, $options);
-        $this->redis = new \Redis();
+
+        if(isset($this->options['redis']) && $this->options['redis'] instanceof \Redis) {
+            $this->redis = $this->options['redis'];
+        }
     }
 
     /**
@@ -80,7 +83,13 @@ function (array $metric) {
      */
     private function openConnection()
     {
+        if($this->redis != null) {
+            return;
+        }
+
         try {
+            $this->redis = new \Redis();
+
             if ($this->options['persistent_connections']) {
                 @$this->redis->pconnect($this->options['host'], $this->options['port'], $this->options['timeout']);
             } else {

tests/Test/Prometheus/Storage/RedisTest.php

@@ -17,4 +17,30 @@ public function itShouldThrowAnExceptionOnConnectionFailure()
         $redis->flushRedis();
     }
 
+    public function testReuseRedisClient()
+    {
+        $redisClient = $this->getMockBuilder(\Redis::class)->getMock();
+        $redisStorage = new Redis(['redis' => $redisClient]);
+
+        $this->assertAttributeEquals($redisClient, 'redis', $redisStorage);
+
+        $redisClient->expects($this->atLeastOnce())
+            ->method('flushAll');
+
+        $redisClient->expects($this->never())
+            ->method('connect');
+
+        $redisStorage->flushRedis();
+    }
+
+    public function testReuseRedisClientWithDefaultOptions()
+    {
+        $redisClient = $this->getMockBuilder(\Redis::class)->getMock();
+
+        Redis::setDefaultOptions(['redis' => $redisClient]);
+
+        $redisStorage = new Redis();
+
+        $this->assertAttributeEquals($redisClient, 'redis', $redisStorage);
+    }
 }