feat: Put request rate limit at application server (#3857)

* feat: Put request rate limit at application server

This PR contains implementation for a blanket rate limiter at
application server level. Currently the allowed throughput is set high
only to be changed later as per the actual data gathered.

* Simplify implementation

1. Remove shutdown handler to purge rate limiter keys
2. Have separate keys for default and custom(route-based) rate limiters
3. Do not kill default rate limiter because it is not needed anymore due
   to (2) above

* Set 60s as default for rate limiting window

* Fix env types

server/env.ts

@@ -495,6 +495,37 @@ export class Environment {
     process.env.SOURCE_COMMIT || process.env.SOURCE_VERSION
   );
 
+  /**
+   * A boolean switch to toggle the rate limiter
+   * at application web server.
+   */
+  @IsOptional()
+  @IsBoolean()
+  public RATE_LIMITER_ENABLED = this.toBoolean(
+    process.env.RATE_LIMITER_ENABLED ?? "false"
+  );
+
+  /**
+   * Set max allowed requests in a given duration for
+   * default rate limiter to trigger throttling.
+   */
+  @IsOptional()
+  @IsNumber()
+  @CannotUseWithout("RATE_LIMITER_ENABLED")
+  public RATE_LIMITER_REQUESTS =
+    this.toOptionalNumber(process.env.RATE_LIMITER_REQUESTS) ?? 5000;
+
+  /**
+   * Set fixed duration window(in secs) for
+   * default rate limiter, elapsing which the request
+   * quota is reset(the bucket is refilled with tokens).
+   */
+  @IsOptional()
+  @IsNumber()
+  @CannotUseWithout("RATE_LIMITER_ENABLED")
+  public RATE_LIMITER_DURATION_WINDOW =
+    this.toOptionalNumber(process.env.RATE_LIMITER_DURATION_WINDOW) ?? 60;
+
   private toOptionalString(value: string | undefined) {
     return value ? value : undefined;
   }