fix(dynamo): divide GSI consumed RCU/WCU metrics by period (#368)

Fixes #362

Results in a dashboard widget with a source like:

```
{
    "view": "timeSeries",
    "title": "Read Capacity",
    "region": "us-east-1",
    "metrics": [
        [ { "label": "Consumed", "expression": "consumed_rcu_sum/PERIOD(consumed_rcu_sum)", "region": "us-east-1" } ],
        [ "AWS/DynamoDB", "ConsumedReadCapacityUnits", "GlobalSecondaryIndexName", "SampleTable2-GSI", "TableName", "SampleTable2", { "stat": "Sum", "visible": false, "id": "consumed_rcu_sum", "region": "us-east-1" } ],
        [ ".", "ProvisionedReadCapacityUnits", ".", ".", ".", ".", { "label": "Provisioned", "stat": "Sum", "region": "us-east-1" } ]
    ],
    "yAxis": {
        "left": {
            "min": 0,
            "label": "Count",
            "showUnits": false
        }
    },
    "timezone": "UTC"
}
```

---

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

Author: Eugene Cheung

lib/monitoring/aws-dynamo/DynamoTableGlobalSecondaryIndexMetricFactory.ts

@@ -12,13 +12,15 @@ export interface DynamoTableGlobalSecondaryIndexMetricFactoryProps {
 
 export class DynamoTableGlobalSecondaryIndexMetricFactory {
   protected readonly metricFactory: MetricFactory;
+  protected readonly table: ITable;
   protected readonly dimensionsMap: DimensionsMap;
 
   constructor(
     metricFactory: MetricFactory,
     props: DynamoTableGlobalSecondaryIndexMetricFactoryProps
   ) {
     this.metricFactory = metricFactory;
+    this.table = props.table;
     this.dimensionsMap = {
       TableName: props.table.tableName,
       GlobalSecondaryIndexName: props.globalSecondaryIndexName,
@@ -48,24 +50,28 @@ export class DynamoTableGlobalSecondaryIndexMetricFactory {
   }
 
   metricConsumedReadCapacityUnits() {
-    return this.metricFactory.createMetric(
-      "ConsumedReadCapacityUnits",
-      MetricStatistic.SUM,
-      "Consumed",
-      this.dimensionsMap,
-      undefined,
-      DynamoDbNamespace
+    return this.metricFactory.createMetricMath(
+      "consumed_rcu_sum/PERIOD(consumed_rcu_sum)",
+      {
+        consumed_rcu_sum: this.table.metricConsumedReadCapacityUnits({
+          statistic: MetricStatistic.SUM,
+          dimensionsMap: this.dimensionsMap,
+        }),
+      },
+      "Consumed"
     );
   }
 
   metricConsumedWriteCapacityUnits() {
-    return this.metricFactory.createMetric(
-      "ConsumedWriteCapacityUnits",
-      MetricStatistic.SUM,
-      "Consumed",
-      this.dimensionsMap,
-      undefined,
-      DynamoDbNamespace
+    return this.metricFactory.createMetricMath(
+      "consumed_wcu_sum/PERIOD(consumed_wcu_sum)",
+      {
+        consumed_wcu_sum: this.table.metricConsumedWriteCapacityUnits({
+          statistic: MetricStatistic.SUM,
+          dimensionsMap: this.dimensionsMap,
+        }),
+      },
+      "Consumed"
     );
   }