feat(metrics): add unit None for CloudWatch EMF Metrics (#2904)

Co-authored-by: Andrea Amorosi <[email protected]>

Author: am29d

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

@@ -31,6 +31,7 @@ const MetricUnit = {
   GigabitsPerSecond: 'Gigabits/Second',
   TerabitsPerSecond: 'Terabits/Second',
   CountPerSecond: 'Count/Second',
+  NoUnit: 'None',
 } as const;
 
 const MetricResolution = {