fix(metrics): export middy middleware (#380)

Author: flochaz

Diff for: packages/metrics/src/index.ts

@@ -1,2 +1,3 @@
 export * from './Metrics';
-export * from './MetricsInterface';
+export * from './MetricsInterface';
+export * from './middleware';

Diff for: packages/metrics/tests/unit/middleware/middy.test.ts

@@ -4,8 +4,7 @@
  * @group unit/metrics/middleware
  */
 
-import { logMetrics } from '../../../../metrics/src/middleware';
-import { Metrics, MetricUnits } from '../../../../metrics/src';
+import { Metrics, MetricUnits, logMetrics } from '../../../../metrics/src';
 import middy from '@middy/core';
 // eslint-disable-next-line @typescript-eslint/ban-ts-comment
 // @ts-ignore