fix(MetricFactory): more specific return type for createMetricSearch (#584)

Fixes #583

Also includes minor README fixes noted in that issue.

---

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

Author: Eugene Cheung

API.md

@@ -202,7 +202,7 @@ Multiple metrics cannot be rendered with anomaly detection in a single widget du
 ```ts
 monitorCustom({
   title: "Metric with anomaly detection",
-  metrics: [
+  metricGroups: [
     {
       metric: m1,
       anomalyDetectionStandardDeviationToRender: 3
@@ -216,7 +216,7 @@ Adding an alarm:
 ```ts
 monitorCustom({
   title: "Metric with anomaly detection and alarm",
-  metrics: [
+  metricGroups: [
     {
       metric: m1,
       alarmFriendlyName: "MetricWithAnomalyDetectionAlarm",
@@ -238,7 +238,7 @@ monitorCustom({
 ```ts
 monitorCustom({
   title: "Metric search",
-  metrics: [
+  metricGroups: [
     {
       searchQuery: "My.Prefix.",
       dimensionsMap: {

README.md

@@ -146,7 +146,7 @@ export class MetricFactory {
     period?: Duration,
     region?: string,
     account?: string,
-  ): IMetric {
+  ): MathExpression {
     const finalPeriod =
       period ?? this.globalDefaults.period ?? DefaultMetricPeriod;
     const searchNamespace = this.getNamespaceWithFallback(namespace);