[Main][Task]24499172: add throttle manager doc

docs/ThrottleMgr.md

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+# Microsoft Application Insights JavaScript SDK - Throttle Manager
+
+Throttle Manager component for the Application Insights Javascript SDK.
+
+## Configuration
+
+[`IThrottleMgrConfig`](https://github.com/microsoft/ApplicationInsights-JS/blob/main/shared/AppInsightsCommon/src/Interfaces/IThrottleMgr.ts)
+
+## Examples
+
+### Initialization
+
+The Throttle Manager component is already integrated into AISKU. If you're using AISKU, simply include your throttle manager configuration `throttleMgrCfg` in your root configuration. Otherwise, you'll need to provide `core` for initialization.
+
+```javascript
+import { AppInsightsCore, _eInternalMessageId } from '@microsoft/applicationinsights-core-js"';
+import { Sender } from "@microsoft/applicationinsights-channel-js";
+
+let core = new AppInsightsCore();
+let sender = new Sender();
+
+let defaultThrottleCfg = {
+    disabled: false, /* throttle is enabled. If set to true, no messages will be sent out */
+    limit: {
+        samplingRate: 100, /* sampling rate is 0.01% */
+        maxSendNumber: 1 /* each time, only throttle 1 message */
+    }, /*  limit number/percentage of items sent per time */
+    interval: {
+        monthInterval: 3, /* messages will be sent out every 3 months */
+        dayInterval: undefined,
+        daysOfMonth: [28] /* on date 28th */
+    }, /* frequency of messages that should be sent out  */
+};
+
+let iKeyDeprecaredMsgThrottleCfg = {
+    disabled: false, /* throttle is enabled. If set to true, no messages will be sent out */
+    limit: {
+        samplingRate: 10, /* sampling rate is 0.001% */
+        maxSendNumber: 1 /* each time, only throttle 1 message */
+    }, /*  limit number/percentage of items sent per time */
+    interval: {
+        monthInterval: undefined,
+        dayInterval: 20, /* messages will be sent out every 20 days */
+        daysOfMonth: undefined
+    }, /* frequency of messages that should be sent out */
+};
+const defaultThrottleMsgKey = _eInternalMessageId.DefaultThrottleMsgKey;
+const instrumentationKeyDeprecation = _eInternalMessageId.InstrumentationKeyDeprecation;
+const cdnDeprecation = _eInternalMessageId.CdnDeprecation;
+
+/* if specific message id throttle confid is not defined, default (_eInternalMessageId.DefaultThrottleMsgKey) config will be used */
+let throttleCfg = {
+    [defaultThrottleMsgKey]: defaultThrottleCfg,
+    [instrumentationKeyDeprecation]: iKeyDeprecaredMsgThrottleCfg
+};
+
+let coreCfg = {
+    connectionString: 'test',
+    throttleMgrCfg: throttleCfg
+};
+core.initialize(coreCfg, [sender]);
+const throttleMgr = new ThrottleMgr(core);
+
+throttleMgr.onReadyState(true); /* the actual throttle will only be activated after onReadyState is set to true */
+throttleMgr.isReady(); /* return if onReadyState is set to true */
+
+```
+
+### Throttle Message
+
+When calling `sendMessage()`, if throttling should happen based on throttle config, trace telemetry will be sent out. Otherwise, local storage will be updated to log the details.
+
+```javascript
+/* return if instrumentationKeyDeprecation message should be throttled or not currently based on throttleCfg[instrumentationKeyDeprecation] */
+throttleMgr.canThrottle(instrumentationKeyDeprecation);
+
+/* return if cdnDeprecation message should be throttled or not currently. Since throttle manager config for cdnDeprecation is not defined, therefore defaultThrottleMsgKey config will be used */
+throttleMgr.canThrottle(cdnDeprecation);
+
+/* return if the instrumentationKeyDeprecation message has been triggered or not on the current date. For each key, throttling will only be triggered once per day.*/
+throttleMgr.isTriggered(instrumentationKeyDeprecation);
+
+/* send instrumentationKeyDeprecation message */
+throttleMgr.sendMessage(instrumentationKeyDeprecation, "Instrumentation Key Deprecation Message");
+
+```