Add minimum configurable default tags to PrometheusReporter

[SokolAndrey]

M3Reporter implementation of StatsReporter allows to set some default tags, which will be added to all reported metrics:

        StatsReporter reporter = new M3Reporter.Builder(new InetSocketAddress("localhost", 9052))
                                     .env("testing")
                                     .service("service_name")
                                     .includeHost(false)
                                     .commonTags(ImmutableMap.of("tag1", "42"))
                                     .build();

There is no such functionality in PrometheusReporter. We should add at minimum ability to set env, service and defaultTags