Final answer:
The correct environment variable for defining custom process group metadata is DT_CUSTOM_METADATA.
Step-by-step explanation:
The environment variable used for defining custom process group metadata in monitoring systems such as Dynatrace is DT_CUSTOM_METADATA. This variable allows you to specify key-value pairs that are meaningful to your organization, which can be used for labeling and organizing monitored processes. It's important to set this variable correctly to ensure that the metadata is applied to your process groups as intended.