fix(cli): cdk watch constantly prints 'messages suppressed' (#18486)

Currently if `filterLogEvents` returns a nextToken then we print
`(...messages suppressed...)`. According to the docs "This operation
can return empty results while there are more log events available
through the token.".

This fix adds a condition to only print out the suppression message if
`filterLogEvents` returns non-empty results.

Also updates the message to be more descriptive.

fixes #18451


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk/lib/api/logs/logs-monitor.ts

@@ -182,8 +182,9 @@ export class CloudWatchLogEventMonitor {
         limit: 100,
         startTime: startTime,
       }).promise();
+      const filteredEvents = response.events ?? [];
 
-      for (const event of response.events ?? []) {
+      for (const event of filteredEvents) {
         if (event.message) {
           events.push({
             message: event.message,
@@ -200,9 +201,9 @@ export class CloudWatchLogEventMonitor {
       // if we have > 100 events let the user know some
       // messages have been supressed. We are essentially
       // showing them a sampling (10000 events printed out is not very useful)
-      if (response.nextToken) {
+      if (filteredEvents.length > 0 && response.nextToken) {
         events.push({
-          message: '(...messages supressed...)',
+          message: '>>> `watch` shows only the first 100 log messages - the rest have been truncated...',
           logGroupName,
           timestamp: new Date(endTime),
         });

packages/aws-cdk/test/api/logs/logs-monitor.test.ts

@@ -55,7 +55,7 @@ test('continue to the next page if it exists', async () => {
     `[${blue('loggroup')}] ${yellow(HUMAN_TIME)} message`,
   );
   expect(stderrMock.mock.calls[1][0]).toContain(
-    `[${blue('loggroup')}] ${yellow(new Date(T100).toLocaleTimeString())} (...messages supressed...)`,
+    `[${blue('loggroup')}] ${yellow(new Date(T100).toLocaleTimeString())} >>> \`watch\` shows only the first 100 log messages - the rest have been truncated...`,
   );
 });