fix(cmon): Use the correct 'layer' label

Node connection error events were being emitted with a lable named
'chainId', which was not defined on the metrics spec.
This was causing nodeConnectionFailures to be treated as unhandled
errors.

.changeset/old-poets-camp.md

+---
+'@eth-optimism/chain-mon': patch
+---
+
+Fixes a bug in the wd-mon service where a node connection failure event was not handled correctly

packages/chain-mon/src/wd-mon/service.ts

@@ -142,7 +142,7 @@ export class WithdrawalMonitor extends BaseServiceV2<Options, Metrics, State> {
         section: 'getBlockNumber',
       })
       this.metrics.nodeConnectionFailures.inc({
-        chainId: this.state.messenger.l1ChainId,
+        layer: 'l1',
         section: 'getBlockNumber',
       })
       await sleep(this.options.sleepTimeMs)