feat(cloudwatch): add addMinIncomingLogsAlarm (#310)

Add ability to alarm on minimum number from `IncomingLogEvents` metric for a given CloudWatch log group. This is useful for detecting issues that would surface as missing incoming logs as a symptom (e.g., improper permissions, service outage/configuration issue, etc.).

All available metrics: https://docs.aws.amazon.com/AmazonCloudWatch/latest/logs/CloudWatch-Logs-Monitoring-CloudWatch-Metrics.html

---

_By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license_

README.md

@@ -74,7 +74,7 @@ You can browse the documentation at https://constructs.dev/packages/cdk-monitori
 | AWS Billing (`.monitorBilling()`) | AWS account cost | Total cost (anomaly) | [Requires enabling](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/gs_monitor_estimated_charges_with_cloudwatch.html#gs_turning_on_billing_metrics) the **Receive Billing Alerts** option in AWS Console / Billing Preferences |
 | AWS Certificate Manager (`.monitorCertificate()`) | Certificate expiration | Days until expiration | |
 | AWS CloudFront (`.monitorCloudFrontDistribution()`) | TPS, traffic, latency, errors | Error rate, low/high TPS | |
-| AWS CloudWatch Logs (`.monitorLog()`) | Patterns present in the log group | | |
+| AWS CloudWatch Logs (`.monitorLog()`) | Patterns present in the log group | Minimum incoming logs | |
 | AWS CloudWatch Synthetics Canary (`.monitorSyntheticsCanary()`) | Latency, error count/rate | Error count/rate, latency | |
 | AWS CodeBuild (`.monitorCodeBuildProject()`) | Build counts (total, successful, failed), failed rate, duration | Failed build count/rate, duration | |
 | AWS DocumentDB (`.monitorDocumentDbCluster()`) | CPU, throttling, read/write latency, transactions, cursors | CPU | |

lib/monitoring/aws-cloudwatch/CloudWatchLogsMetricFactory.ts

@@ -0,0 +1,38 @@
+import { DimensionsMap } from "aws-cdk-lib/aws-cloudwatch";
+
+import { MetricFactory, MetricStatistic } from "../../common";
+
+const CloudWatchLogsNamespace = "AWS/Logs";
+
+export interface CloudWatchLogsMetricFactoryProps {
+  /**
+   * Name of the log group to monitor.
+   */
+  readonly logGroupName: string;
+}
+
+export class CloudWatchLogsMetricFactory {
+  private readonly metricFactory: MetricFactory;
+  private readonly dimensionsMap: DimensionsMap;
+
+  constructor(
+    metricFactory: MetricFactory,
+    props: CloudWatchLogsMetricFactoryProps
+  ) {
+    this.metricFactory = metricFactory;
+    this.dimensionsMap = {
+      LogGroupName: props.logGroupName,
+    };
+  }
+
+  metricIncomingLogEvents() {
+    return this.metricFactory.createMetric(
+      "IncomingLogEvents",
+      MetricStatistic.N,
+      "Logs",
+      this.dimensionsMap,
+      undefined,
+      CloudWatchLogsNamespace
+    );
+  }
+}