Added option connect(MongoDatabase mongoDB) to builder:

[StephanRichter]

This option allows to re-use an already established database connection
instead of building a new one in conjunctino with the Reporter.Builder.

This will allow something like:

// existing code
ServerAddress addr = new ServerAddress("localhost", 27017);
MongoCredential credential = MongoCredential.createCredential("user", "coll", "pass".toCharArray());
MongoClientOptions options = new MongoClientOptions.Builder().build();
MongoClient mongoClient = new MongoClient(addr, credential, options);
MongoDatabase mongoDB = mongoClient.getDatabase("coll");

// ...

// metrics added to code already unsing database
MongoDBReporter reporter = MongoDBReporter.forRegistry(metrics).connect(mongoDB).build();