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/unit/test_base/test_subscriber_graceful_shutdown.py

@@ -412,4 +412,4 @@ async def test_subscriber_multiple_subscribers():
     msg1 = await queue1.get()
     msg_all = await queue_all.get()
     assert msg1 == {"data": "broadcast"}
-    assert msg_all == {"data": "broadcast"}
+    assert msg_all == {"data": "broadcast"}