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);
+        });
+    });
+});

test/prometheus-server.js

@@ -0,0 +1,65 @@
+const assert = require('assert');
+const http = require('http');
+const server = require('../lib/prometheus-server');
+const PrometheusConfig = require('../lib/configuration/prometheusconfig').PrometheusConfig;
+
+describe('prometheus-server', () => {
+    before(done => {
+        let inst = server.init(new PrometheusConfig({
+            prometheus: {
+                enabled: true,
+                port: 19820,
+                host: '127.0.0.1',
+                path: '/metrics'
+            }
+        }));
+        inst.once('listening', () => done());
+    });
+
+    function checkReq(options, done) {
+        const req = http.request({
+            method: options.method,
+            host: '127.0.0.1',
+            port: 19820,
+            path: options.path
+        }, res => {
+            assert.strictEqual(res.statusCode, options.expectedStatusCode);
+            assert.strictEqual(res.headers['content-type'], options.expectedContentType);
+            res.on('data', () => {});
+            res.on('end', () => done());
+        });
+
+        req.end();
+    }
+
+    it('rejects a non-GET request', done => {
+        checkReq({
+            method: 'POST',
+            path: '/metrics',
+            expectedStatusCode: 400,
+            expectedContentType: 'text/plain'
+        }, done);
+    });
+
+    it('rejects a request for the wrong path', done => {
+        checkReq({
+            method: 'GET',
+            path: '/qwerty',
+            expectedStatusCode: 400,
+            expectedContentType: 'text/plain'
+        }, done);
+    });
+
+    it('accepts a request for the configured path', done => {
+        checkReq({
+            method: 'GET',
+            path: '/metrics',
+            expectedStatusCode: 200,
+            expectedContentType: 'text/plain; version=0.0.4; charset=utf-8'
+        }, done);
+    });
+
+    after(() => {
+        server.shutdown();
+    });
+});