feat(ec2): Also plot EBS disk metrics when NITRO instance type is specified (#321)

EC2 provides different metrics for Nitro-based instances. These metrics have the prefix `EBS`.

These instances either have the `Disk*` or `EBSDisk*` metrics, never both. Because of this I thought in using [metric math](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/using-metric-math.html) where if any of the time series is empty, it would be removed, and then it would display a single metric. This gets trickier when monitoring an ASG because it can have both metrics there, so what should be done with the 2 time series? An average of them? If you think this would be a better idea, please advise so I change the implementation.

---

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

Author: edisongustavo

API.md

@@ -161,7 +161,7 @@ export class EC2MetricFactory {
    * This can be used to determine the speed of the application.
    */
   metricAverageDiskReadBytes() {
-    return this.createMetrics("DiskReadBytes", MetricStatistic.AVERAGE);
+    return this.createDiskMetrics("ReadBytes", MetricStatistic.AVERAGE);
   }
 
   /**
@@ -170,21 +170,21 @@ export class EC2MetricFactory {
    * This can be used to determine the speed of the application.
    */
   metricAverageDiskWriteBytes() {
-    return this.createMetrics("DiskWriteBytes", MetricStatistic.AVERAGE);
+    return this.createDiskMetrics("WriteBytes", MetricStatistic.AVERAGE);
   }
 
   /**
    * Completed read operations from all instance store volumes available to the instance in a specified period of time.
    */
   metricAverageDiskReadOps() {
-    return this.createMetrics("DiskReadOps", MetricStatistic.AVERAGE);
+    return this.createDiskMetrics("ReadOps", MetricStatistic.AVERAGE);
   }
 
   /**
    * Completed write operations to all instance store volumes available to the instance in a specified period of time.
    */
   metricAverageDiskWriteOps() {
-    return this.createMetrics("DiskWriteOps", MetricStatistic.AVERAGE);
+    return this.createDiskMetrics("WriteOps", MetricStatistic.AVERAGE);
   }
 
   /**
@@ -203,6 +203,33 @@ export class EC2MetricFactory {
     return this.createMetrics("NetworkOut", MetricStatistic.AVERAGE);
   }
 
+  private createDiskMetrics(metricName: string, statistic: MetricStatistic) {
+    const classicMetrics = this.strategy.createMetrics(
+      this.metricFactory,
+      `Disk${metricName}`,
+      statistic
+    );
+    const ebsMetrics = this.strategy.createMetrics(
+      this.metricFactory,
+      `EBS${metricName}`,
+      statistic
+    );
+
+    return classicMetrics.map((classic, i) => {
+      const ebs = ebsMetrics[i];
+      const usingMetrics: Record<string, IMetric> = {};
+      const classicId = `${metricName.toLowerCase()}_classic_${i}`;
+      const ebsId = `${metricName.toLowerCase()}_ebs_${i}`;
+      usingMetrics[classicId] = classic;
+      usingMetrics[ebsId] = ebs;
+      return this.metricFactory.createMetricMath(
+        `AVG(REMOVE_EMPTY([${classicId}, ${ebsId}]))`,
+        usingMetrics,
+        `Disk${metricName}`
+      );
+    });
+  }
+
   private createMetrics(metricName: string, statistic: MetricStatistic) {
     return this.strategy.createMetrics(
       this.metricFactory,