chore: stop mypy from complaining about the Metric class (aws#20369)

Fixes: aws#20219

----

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packages/@aws-cdk/aws-cloudwatch/lib/metric.ts

@@ -279,6 +279,9 @@ export class Metric implements IMetric {
   /** Region which this metric comes from. */
   public readonly region?: string;
 
+  /** Warnings attached to this metric. */
+  public readonly warnings?: string[];
+
   constructor(props: MetricProps) {
     this.period = props.period || cdk.Duration.minutes(5);
     const periodSec = this.period.toSeconds();
@@ -295,6 +298,7 @@ export class Metric implements IMetric {
     this.unit = props.unit;
     this.account = props.account;
     this.region = props.region;
+    this.warnings = undefined;
   }
 
   /**
@@ -889,4 +893,4 @@ function matchAll(x: string, re: RegExp): RegExpMatchArray[] {
     ret.push(m);
   }
   return ret;
-}
+}