Added health metrics for application load and database load (#2257)

* Added health metrics for application load

* Added health metrics for application load

* Fix unit tests

* Added more metrics

* Added few comments

* Added DatabaseLoad Scenario and address comments

* Fix unit tests

* fix unit tests

* Fix unit tests

* fix unit tests

* fix the mock

* Fix unit tests

src/Metrics/Constants.ts

@@ -0,0 +1,16 @@
+import { ApiType } from "Common/Constants";
+import { Platform } from "ConfigContext";
+
+// Metric scenarios represent lifecycle checkpoints we measure.
+export enum MetricScenario {
+  ApplicationLoad = "ApplicationLoad",
+  DatabaseLoad = "DatabaseLoad",
+}
+
+// Generic metric emission event describing scenario outcome.
+export interface MetricEvent {
+  readonly platform: Platform;
+  readonly api: ApiType;
+  readonly scenario: MetricScenario;
+  readonly healthy: boolean;
+}