Which metric in Datadog indicates on which hosts APM is running?

The chosen answer, datadog.apm.host_instance, is correct because it directly indicates the instances of hosts where the Application Performance Monitoring (APM) agent is running. This metric provides crucial information about the active hosts that are being monitored by the APM, enabling users to understand which servers are engaged in tracing and performance measurement for their applications.

This metric would often be used to correlate application performance data with infrastructure monitoring, allowing users to identify potential performance bottlenecks and optimize their resources effectively. By tracking this specific metric, teams can ensure that they have visibility into the configurations and status of the APM agents across their infrastructure, supporting more informed troubleshooting and optimization efforts.

Other options represent different aspects of telemetry or agent status but do not specifically indicate on which hosts APM is active. For instance, metrics that focus on status or general agent performance don't provide the specific host-level visibility that is critical for APM. Therefore, the correct focus on the individual hosts is what makes datadog.apm.host_instance the most appropriate choice for identifying where APM is operational.