feat(metrics): thread safety with concurrent collections and isolation for metrics (#1080)

Author: hjgraca

libraries/src/AWS.Lambda.Powertools.Metrics/InternalsVisibleTo.cs

@@ -17,4 +17,5 @@
 
 [assembly: InternalsVisibleTo("AWS.Lambda.Powertools.Metrics.Tests")]
 [assembly: InternalsVisibleTo("AWS.Lambda.Powertools.Metrics.AspNetCore")]
-[assembly: InternalsVisibleTo("AWS.Lambda.Powertools.Metrics.AspNetCore.Tests")]
+[assembly: InternalsVisibleTo("AWS.Lambda.Powertools.Metrics.AspNetCore.Tests")]
+[assembly: InternalsVisibleTo("AWS.Lambda.Powertools.ConcurrencyTests")]