Add metric unit

Signed-off-by: Gabriel Dugny <gabriel.dugny@believe.com>

.changeset/solid-bats-flash.md

@@ -2,4 +2,4 @@
 '@backstage/plugin-notifications-backend-module-slack': patch
 ---
 
-The Slack notification processor now uses the `MetricsService` to create metrics, providing plugin-scoped attribution.
+The Slack notification processor now uses the `MetricsService` to create metrics, providing plugin-scoped attribution. `{message}` unit has also been added.

plugins/notifications-backend-module-slack/src/lib/SlackNotificationProcessor.ts

@@ -169,12 +169,14 @@ export class SlackNotificationProcessor implements NotificationProcessor {
       'notifications.processors.slack.sent.count',
       {
         description: 'Number of messages sent to Slack successfully',
+        unit: '{message}',
       },
     );
     this.messagesFailed = metrics.createCounter(
       'notifications.processors.slack.error.count',
       {
         description: 'Number of messages that failed to send to Slack',
+        unit: '{message}',
       },
     );