/usr/share/diagnostic_msgs/msg/DiagnosticArray.msg is in libdiagnostic-msgs-dev 1.12.3-5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # This message is used to send diagnostic information about the state of the robot
Header header #for timestamp
DiagnosticStatus[] status # an array of components being reported on
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