[perfmon] Fix typo in variable name

[gthvn1]

No description provided.

[psafont]

@gthvn1 Tests are failing because perfmon doesn't have any tests. This means that currently perfmon can't be modified without spending the effort to add them

[gthvn1]

Oh. I understood that python3/tests/test_extauth_hook_AD.py and nbd_client_manager:nbd_client_manager.py failed. So it was not related to my modification. And I see the coverage issue but I thought it was only a warning.
Ok so I guess that need to have a closer look to understand how to add a test ;)

[gthvn1]

In fact I see in the logs:

...
python3/tests/test_perfmon.py::TestGetFsUsage::test_get_percent_fs_usage PASSED [ 40%]
python3/tests/test_perfmon.py::TestGetFsUsage::test_get_percent_log_fs_usage PASSED [ 41%]
python3/tests/test_perfmon.py::TestGetFsUsage::test_get_percent_log_fs_usage_same_file_system PASSED [ 42%]
python3/tests/test_perfmon.py::TestGetMemUsage::test_get_percent_mem_usage PASSED [ 43%]
python3/tests/test_perfmon.py::TestGetMemUsage::test_get_percent_mem_usage_exception PASSED [ 43%]
python3/tests/test_perfmon.py::TestGetPercentSRUsage::test_get_percent_sr_usage_correct_input PASSED [ 44%]
python3/tests/test_perfmon.py::TestGetPercentSRUsage::test_get_percent_sr_usage_exception_handling PASSED [ 45%]
...

So it looks like perfmon has some tests.
But when I run tests locally (I just run pytest tests) I have an issue with tests/observer/it_traces.py.

>           assert span.__name__ == "span_of_tracers"
E           AssertionError: assert '_span_noop' == 'span_of_tracers'
E
E             - span_of_tracers
E             + _span_noop

tests/observer/it_traces.py:73: AssertionError
---------------------------------------------------------------------------------------------- Captured stdout call ----------------------------------------------------------------------------------------------
Hello, I am a print() in traced_script.py.
----------------------------------------------------------------------------------------------- Captured log call ------------------------------------------------------------------------------------------------
DEBUG    __main__:observer.py:400 configs = ['/home/gthouvenin/git/xen-api/python3/tests/observer/observer.conf']
ERROR    __main__:observer.py:147 missing opentelemetry dependencies: No module named 'opentelemetry'
============================================================================================ short test summary info =============================================================================================
FAILED tests/observer/it_traces.py::it_creates_a_tracer - AssertionError: assert '_span_noop' == 'span_of_tracers'
========================================================================================= 1 failed, 109 passed in 0.32s ==========================================================================================