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.
