bug: metric tracing crashes node

[deepfire]

The node has a non-trivial probability of crashing during startup:

cardano-node: ExceptionInLinkedThread "ThreadId 42" The name ""cardano.node.blockContext"" is already taken by a metric.
CallStack (from HasCallStack):
  error, called at ./System/Metrics.hs:214:5 in ekg-core-0.1.1.7-FjoslY1tzknIAl90c73kOZ:System.Metrics

..or something similar -- the metric name varies.

Repro:

• cardano-node master
• make default-autostay, which starts six nodes -- and that should trigger it, at least for one of them.
• If not -- retry until success : -)

[deepfire]

Running a make default-prof cluster sheds further light:

cardano-node: ExceptionInLinkedThread "ThreadId 35" The name ""cardano.node.blockContext"" is already taken by a metric.
CallStack (from HasCallStack):
  error, called at ./System/Metrics.hs:214:5 in ekg-core-0.1.1.7-FjoslY1tzknIAl90c73kOZ:System.Metrics
CallStack (from -prof):
  System.Metrics.registerGauge (System/Metrics.hs:(172,1)-(173,39))
  System.Metrics.createGauge (System/Metrics.hs:(303,1)-(306,16))
  Cardano.Logging.Tracer.EKG.ekgTracer (src/Cardano/Logging/Tracer/EKG.hs:(26,1)-(94,61))
  Cardano.Node.Tracing.API.initTraceDispatcher (src/Cardano/Node/Tracing/API.hs:(61,1)-(120,28))
  Ouroboros.Consensus.Node.Tracers.forgeTracer (src/Ouroboros/Consensus/Node/Tracers.hs:62:5-15)
  Ouroboros.Consensus.BlockchainTime.API.knownSlotWatcher (src/Ouroboros/Consensus/BlockchainTime/API.hs:(64,1)-(77,40))
  Control.Concurrent.Async.async (Control/Concurrent/Async.hs:264:1-35)
  Ouroboros.Consensus.Util.ResourceRegistry.allocateEither (src/Ouroboros/Consensus/Util/ResourceRegistry.hs:(979,1)-(1011,9))
  Ouroboros.Consensus.Util.ResourceRegistry.allocate (src/Ouroboros/Consensus/Util/ResourceRegistry.hs:(968,1)-(969,72))
  Ouroboros.Consensus.Util.ResourceRegistry.forkThread (src/Ouroboros/Consensus/Util/ResourceRegistry.hs:(1149,1)-(1186,35))
  Ouroboros.Consensus.Util.ResourceRegistry.forkLinkedThread (src/Ouroboros/Consensus/Util/ResourceRegistry.hs:(1261,1)-(1268,12))
  Ouroboros.Consensus.Util.STM.forkLinkedWatcher (src/Ouroboros/Consensus/Util/STM.hs:(163,1)-(164,56))
  Ouroboros.Consensus.NodeKernel.initNodeKernel (src/Ouroboros/Consensus/NodeKernel.hs:(146,1)-(178,7))
  Ouroboros.Consensus.Util.ResourceRegistry.withRegistry (src/Ouroboros/Consensus/Util/ResourceRegistry.hs:649:1-54)
  Ouroboros.Consensus.Node.Recovery.runWithCheckedDB (src/Ouroboros/Consensus/Node/Recovery.hs:(97,1)-(134,39))
  Ouroboros.Consensus.Node.DbLock.withLockDB_ (src/Ouroboros/Consensus/Node/DbLock.hs:(57,1)-(80,53))
  Ouroboros.Consensus.Node.DbLock.withLockDB (src/Ouroboros/Consensus/Node/DbLock.hs:(27,1)-(32,19))
  Ouroboros.Consensus.Node.stdWithCheckedDB (src/Ouroboros/Consensus/Node.hs:(504,1)-(517,35))
  Ouroboros.Consensus.Node.stdLowLevelRunNodeArgsIO (src/Ouroboros/Consensus/Node.hs:(752,1)-(851,60))
  Ouroboros.Consensus.Node.runWith (src/Ouroboros/Consensus/Node.hs:(283,1)-(491,35))
  Ouroboros.Consensus.Node.run (src/Ouroboros/Consensus/Node.hs:267:1-73)
  Cardano.Node.Handlers.Shutdown.withShutdownHandling (src/Cardano/Node/Handlers/Shutdown.hs:(114,1)-(130,65))
  Cardano.Node.Run.runNode (src/Cardano/Node/Run.hs:(110,1)-(152,58))
  Main.runRunCommand (app/cardano-node.hs:99:1-40)
  Cardano.Node.Handlers.TopLevel.toplevelExceptionHandler (src/Cardano/Node/Handlers/TopLevel.hs:(67,1)-(114,42))
  Main.main (app/cardano-node.hs:(27,1)-(59,44))

[deepfire]

Fixed in #4108