Added metric for tracking last scanned block by ritual tracker scanner task.

Added test.

nucypher/blockchain/eth/trackers/dkg.py

@@ -4,6 +4,7 @@ import time
 from typing import Callable, List, Optional, Tuple
 
 import maya
+from prometheus_client import REGISTRY, Gauge
 from twisted.internet import threads
 from web3.datastructures import AttributeDict
 
@@ -66,6 +67,13 @@ class EventScannerTask(SimpleTask):
             self.start(now=False)  # take a breather
 
 
+LAST_SCANNED_BLOCK_METRIC = Gauge(
+    "ritual_events_last_scanned_block_number",
+    "Last scanned block number for ritual events",
+    registry=REGISTRY,
+)
+
+
 class ActiveRitualTracker:
 
     MAX_CHUNK_SIZE = 10000
@@ -417,14 +425,16 @@ class ActiveRitualTracker:
         Because there might have been a minor Ethereum chain reorganisations since the last scan ended,
         we need to discard the last few blocks from the previous scan results.
         """
-        self.scanner.delete_potentially_forked_block_data(
-            self.state.get_last_scanned_block() - self.scanner.chain_reorg_rescan_window
-        )
+        last_scanned_block = self.scanner.get_last_scanned_block()
+        LAST_SCANNED_BLOCK_METRIC.set(last_scanned_block)
 
-        if self.scanner.get_last_scanned_block() == 0:
+        if last_scanned_block == 0:
             # first run so calculate starting block number based on dkg timeout
             suggested_start_block = self._get_first_scan_start_block_number()
         else:
+            self.scanner.delete_potentially_forked_block_data(
+                last_scanned_block - self.scanner.chain_reorg_rescan_window
+            )
             suggested_start_block = self.scanner.get_suggested_scan_start_block()
 
         end_block = self.scanner.get_suggested_scan_end_block()

tests/acceptance/actors/test_dkg_ritual.py

@@ -197,6 +197,11 @@ def test_ursula_ritualist(
         for ursula in cohort:
             assert ursula.dkg_storage.get_transcript(RITUAL_ID) is not None
 
+        last_scanned_block = REGISTRY.get_sample_value(
+            "ritual_events_last_scanned_block_number"
+        )
+        assert last_scanned_block > 0
+
     def test_encrypt(_):
         """Encrypts a message and returns the ciphertext and conditions"""
         print("==================== DKG ENCRYPTION ====================")