test: Add comments to the setup hook for the client side metrics tests (#1667)

## Description

This change is a follow-up to https://togithub.com/googleapis/nodejs-bigtable/pull/1662/files where we discussed maybe a comment would be helpful for this test.

## Impact

Better test readability

## Testing

Just comments added

Author: danieljbruce

system-test/client-side-metrics-all-methods.ts

@@ -357,6 +357,18 @@ describe('Bigtable/ClientSideMetricsAllMethods', () => {
   let defaultProjectId: string;
 
   before(async () => {
+    /*
+    For both the default project and the secondary project we need to create
+    instances with some data in them so that the tests can collect all the
+    metrics they would normally collect in a typical situation and compare
+    those metrics against expected results.
+
+    We need tests like "should send the metrics to Google Cloud Monitoring for a
+    ReadRows call with a second project" that work with a second project because
+    we want to ensure that when a user specifies a second project that the
+    metrics actually get written for that other project instead of the default
+    project.
+     */
     for (const bigtable of [
       new Bigtable(),
       new Bigtable({projectId: SECOND_PROJECT_ID}),

system-test/client-side-metrics.ts

@@ -334,6 +334,18 @@ describe('Bigtable/ClientSideMetrics', () => {
   let defaultProjectId: string;
 
   before(async () => {
+    /*
+    For both the default project and the secondary project we need to create
+    instances with some data in them so that the tests can collect all the
+    metrics they would normally collect in a typical situation and compare
+    those metrics against expected results.
+
+    We need tests like "should send the metrics to Google Cloud Monitoring for a
+    ReadRows call with a second project" that work with a second project because
+    we want to ensure that when a user specifies a second project that the
+    metrics actually get written for that other project instead of the default
+    project.
+     */
     for (const bigtable of [
       new Bigtable(),
       new Bigtable({projectId: SECOND_PROJECT_ID}),