Raise an exception when another is running

[benoit-cty]

Problem

Currently, when running codecarbon monitor while another instance is active, the CLI displays "Another instance of codecarbon is already running" but continues its infinite loop instead of terminating. This creates a confusing user experience.

Solution

Modify the behavior to raise an exception when detecting another running instance, forcing the program to terminate unless the calling code explicitly handles this case.
This makes the error condition more explicit and helps prevent unintended parallel executions of codecarbon monitoring.
Please review and provide feedback on this approach.

[benoit-cty]

After a discussion, we prefer to keep this behavior : let the user know in log that CodeCarbon will not work and do not stop his process.

Here is an code sample if a user want to know about CodeCarbon not doing the measure:

  with EmissionsTracker() as tracker:
       # Infinite loop
       while True:
           if (
               hasattr(tracker, "_another_instance_already_running")
               and tracker._another_instance_already_running
           ):
               print("Another instance of CodeCarbon is already running. Exiting.")
               break
           # The compute code goes here
           time.sleep(300)