Add compression and histogram metric type for Prometheus (#987)

Fixes #932
Also fixes #966

Decided not to add Summary type because it has the same functionality as Histogram but makes more calculations on client side (Clio side). See https://prometheus.io/docs/practices/histograms for detailed comparison.

example-config.json

@@ -101,7 +101,10 @@
             "log_level": "trace"
         }
     ],
-    "prometheus_enabled": true,
+    "prometheus": {
+        "enabled": true,
+        "compress_reply": true,
+    },
     "log_level": "info",
     // Log format (this is the default format)
     "log_format": "%TimeStamp% (%SourceLocation%) [%ThreadID%] %Channel%:%Severity% %Message%",