Add host metrics receiver warning for DDOT Collector (#35897)

* Add warning that host metrics receiver is not needed for DDOT Collector

* Change alert warning to alert danger in host_metrics.md

Author: brett0000FF

content/en/opentelemetry/integrations/host_metrics.md

@@ -12,6 +12,8 @@ further_reading:
 
 {{< img src="/opentelemetry/collector_exporter/host_metrics.png" alt="OpenTelemetry host metrics dashboard" style="width:100%;" >}}
 
+<div class="alert alert-danger">The host metrics receiver is only required for the standalone OpenTelemetry Collector. If you are using the <a href="/opentelemetry/setup/ddot_collector/">DDOT Collector</a>, the Datadog Agent already collects host metrics.</div>
+
 To collect system metrics such as CPU, disk, and memory usage, enable the [host metrics receiver][1] in your Collector. 
 
 For more information, including supported operating systems, see the OpenTelemetry project documentation for the [host metrics receiver][1].