fix(createStatsDClient): Return full StatsD type (#51)

BREAKING CHANGE: the `StatsD` class must be passed as the first argument
of the factory function. The return type will reflect the input type, so
you don't need to perform a type assertion `as StatsD` anymore.

```diff
import { StatsD } from 'hot-shots';
import { createStatsDClient } from 'seek-datadog-custom-metrics';

- export const metricsClient = createStatsDClient(config, errHandler) as StatsD;
+ export const metricsClient = createStatsDClient(StatsD, config, errHandler);
```

README.md

@@ -43,6 +43,7 @@ These tags are consistent with tags sent by [Gantry](https://github.com/SEEK-Job
 This is intended for containerized services, particularly those deployed with [Gantry](https://github.com/SEEK-Jobs/gantry).
 
 ```typescript
+import { StatsD } from 'hot-shots';
 import { createStatsDClient } from 'seek-datadog-custom-metrics';
 
 // Expects `name`, `version`, `environment` and `metricsServer` properties
@@ -55,7 +56,7 @@ const errorHandler = (err: Error) => {
 };
 
 // Returns a standard hot-shots StatsD instance
-const metricsClient = createStatsDClient(config, errorHandler);
+const metricsClient = createStatsDClient(StatsD, config, errorHandler);
 ```
 
 ### `createCloudWatchClient`

src/createStatsDClient.test.ts

@@ -1,9 +1,15 @@
+import { StatsD } from 'hot-shots';
+
 import createStatsDClient from './createStatsDClient';
 
 describe('createStatsDClient', () => {
   it('should create a new mock client', () => {
     expect(
-      createStatsDClient({ name: 'test', environment: 'jest', version: '0' }),
+      createStatsDClient(StatsD, {
+        name: 'test',
+        environment: 'jest',
+        version: '0',
+      }),
     ).toBeInstanceOf(Object);
   });
 });

src/createStatsDClient.ts

@@ -1,9 +1,23 @@
-import type { StatsD } from 'hot-shots';
-
 import AppConfig from './AppConfig';
 import MetricsClient from './MetricsClient';
 import globalTags from './globalTags';
 
+interface InternalStatsD extends MetricsClient {
+  socket?: {
+    close(callback?: () => void): void;
+  };
+}
+
+interface StatsD<T extends InternalStatsD> {
+  new (options?: {
+    mock?: boolean;
+    host?: string;
+    errorHandler?: (err: Error) => void;
+    prefix?: string;
+    globalTags?: Record<string, string> | string[];
+  }): T;
+}
+
 /**
  * Configuration for building a StatsD client
  */
@@ -23,20 +37,16 @@ export interface StatsDConfig extends AppConfig {
  * The returned client is configured to use a common tagging convention based
  * on the application's name, environment and version.
  *
+ * @param StatsD       - StatsD class from `hot-shots`
  * @param config       - Application configuration
  * @param errorHandler - Optional error handler function
  */
-export default (
+export default <T extends InternalStatsD>(
+  StatsD: StatsD<T>,
   config: StatsDConfig,
   errorHandler?: (err: Error) => void,
-): MetricsClient => {
-  // Avoid a hard dependency on `hot-shots` for e.g. Lambda CloudWatch users
-  // This severely angers TypeScript in multiple ways.
-
-  // eslint-disable-next-line
-  const StatsDClass = require('hot-shots').StatsD as typeof StatsD;
-
-  const client = new StatsDClass({
+): T => {
+  const client = new StatsD({
     // Disable ourselves if there's no configured metrics server
     mock: !config.metricsServer,
     host: config.metricsServer,

src/createTimedSpan.test.ts

@@ -1,7 +1,9 @@
+import { StatsD } from 'hot-shots';
+
 import createStatsDClient from './createStatsDClient';
 import createTimedSpan from './createTimedSpan';
 
-const metricsClient = createStatsDClient({
+const metricsClient = createStatsDClient(StatsD, {
   name: 'jest',
   version: '0.0.1',
   environment: 'dev',