Begin prometheus integration

Add a dependency on `prom-client` and emit a basic latency metric for
testing purposes.  Add a new configuration file for enabling/disabling
prometheus exporter and configuring the listen address.

src/configuration/prometheusconfig.js

@@ -0,0 +1,23 @@
+class PrometheusConfig {
+    constructor(config = { prometheus: { enabled: false } }) {
+        this.config = config.prometheus;
+    }
+
+    isEnabled() {
+        return this.config.enabled;
+    }
+
+    getPort() {
+        return this.config.port;
+    }
+
+    getHost() {
+        return this.config.host;
+    }
+
+    getPath() {
+        return this.config.path;
+    }
+}
+
+export { PrometheusConfig };