Fix poa chains (#22)

Author: namikmesic

src/collectors/evm.py

@@ -1,5 +1,6 @@
 from settings import cfg, logger
 from web3 import Web3
+from web3.exceptions import ExtraDataLengthError
 import asyncio
 from helpers import strip_url, check_protocol, generate_labels_from_metadata
 from collectors.ws import websocket_collector
@@ -23,22 +24,28 @@ def __init__(self, rpc_metadata):
                                                     })
             self.ws_collector.setDaemon(True)
             self.ws_collector.start()
+            self.record_difficulty = True
 
         else:
             logger.error("Please provide wss/ws endpoint for {}".format(strip_url(self.url)))
             exit(1)
 
     def probe(self) -> results:
-        results.register(self.url, self.labels_values)
         try:
             if self.client.isConnected():
+                results.register(self.url, self.labels_values)
                 results.record_health(self.url, True)
                 results.record_head_count(self.url, self.ws_collector.message_counter)
                 results.record_disconnects(self.url, self.ws_collector.disconnects_counter)
                 results.record_latency(self.url, self.ws_collector.get_latency())
                 results.record_block_height(self.url, self.client.eth.block_number)
-                results.record_total_difficulty(self.url, self.client.eth.get_block('latest')['totalDifficulty'])
-                results.record_difficulty(self.url, self.client.eth.get_block('latest')['difficulty'])
+                try:
+                    if self.record_difficulty:
+                        results.record_total_difficulty(self.url, self.client.eth.get_block('latest')['totalDifficulty'])
+                        results.record_difficulty(self.url, self.client.eth.get_block('latest')['difficulty'])
+                except ExtraDataLengthError:
+                    logger.info("It looks like this is a POA chain, and does not use difficulty anymore. Collector will ignore difficulty metric from this point on")
+                    self.record_difficulty = False
                 results.record_gas_price(self.url, self.client.eth.gas_price)
                 results.record_max_priority_fee(self.url, self.client.eth.max_priority_fee)
                 results.record_client_version(self.url, self.client.clientVersion)