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.

test/configuration/prometheusconfig.js

@@ -0,0 +1,10 @@
+const assert = require('assert');
+const PrometheusConfig = require('../../lib/configuration/prometheusconfig').PrometheusConfig;
+
+describe('PrometheusConfig', () => {
+    describe('#constructor', () => {
+        it('defaults to enabled=false', () => {
+            assert.strictEqual(new PrometheusConfig().isEnabled(), false);
+        });
+    });
+});