integrations: Add screenshot configs for monitoring integrations.

that do not have fixtures.

zerver/lib/integrations.py

@@ -74,6 +74,7 @@ FIXTURELESS_INTEGRATIONS_WITH_SCREENSHOTS: list[str] = [
     "jenkins",
     "mastodon",
     "mercurial",
+    "nagios",
     "notion",
     "openshift",
     "perforce",

zerver/webhooks/fixtureless_integrations.py

@@ -119,6 +119,15 @@ MERCURIAL = ScreenshotContent(
 """,
 )
 
+NAGIOS = ScreenshotContent(
+    topic="service Remote Load on myserver.example.com",
+    content="""**PROBLEM**: service is CRITICAL
+~~~~
+CRITICAL - load average: 7.49, 8.20, 4.72
+~~~~
+""",
+)
+
 NOTION = ScreenshotContent(
     topic=f"{PROJECT_NAME} Release {VERSION_NUMBER}",
     content=f"""**{BO_NAME}** [commented]() on: