Fix test suite Prometheus registry pollution and remove default (#806)

Fix test suite Prometheus registry pollution and remove default coverage

The test_metrics.py fixture (added in 4e63dbda) deleted
class-level metric singleton attributes without restoring them. This
stripped the hasattr() guards that prevent duplicate Prometheus
registration, so every subsequent Processor construction in the test
run raised ValueError: Duplicated timeseries. Fix by saving and
restoring the original attributes around each test.

Remove --cov=trustgraph from pytest.ini defaults — the coverage
import hooks interact badly with the trustgraph namespace package,
causing duplicate class loading. Coverage can still be requested
explicitly via the command line.

tests/pytest.ini

@@ -4,14 +4,11 @@ python_paths = .
 python_files = test_*.py
 python_classes = Test*
 python_functions = test_*
-addopts = 
+addopts =
     -v
     --tb=short
     --strict-markers
     --disable-warnings
-    --cov=trustgraph
-    --cov-report=html
-    --cov-report=term-missing
 #    --cov-fail-under=80
 asyncio_mode = auto
 markers =