[TEST] Only run the multi_observer_test when building with ABI v2

[dbarker]

The MultiObserverTest cases require ABI v2, however ctest attempts to run test even when CMake is configured with ABI v1.

gtest_add_tests() scans source files with regex at configure time, ignoring #ifdef guards. The four MultiObserverTest cases run and fail locally in the devcontainer (ie: ./ci/do_ci.sh cmake.maintainer.async.test).

The following tests FAILED:
        480 - metrics.MultiObserverTest.BasicMultiObservableCallback (Failed)
        481 - metrics.MultiObserverTest.EmptyInstrumentsList (Failed)
        482 - metrics.MultiObserverTest.NonRegisteredInstrument (Failed)
        483 - metrics.MultiObserverTest.CallbackDeregisteredWhenInstrumentDestroyed (Failed)

Changes

• Only add the test when ABI >= 2 in CMakeLists.txt.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.17%. Comparing base (90d9173) to head (7b09373).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3982      +/-   ##
==========================================
- Coverage   90.18%   90.17%   -0.01%     
==========================================
  Files         230      230              
  Lines        7299     7299              
==========================================
- Hits         6582     6581       -1     
- Misses        717      718       +1     

see 1 file with indirect coverage changes

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[marcalff]

LGTM, thanks for the fix.