fix(fabric): Multiple objects of the ledger connector class throws errors with prometheus exporter object

[jagpreetsinghsasan]

Describe the bug
When multiple objects of the ledger-connector-fabric are instantiated, it causes issues with prometheus exporter object initialization.

To Reproduce

Instantiate multiple ledger-connector-fabric objects.
prometheus exporter object throws error stating
Error: A metric with the name process_cpu_user_seconds_total has already been registered.

Expected behavior
Multiple object initialization should occur without any issues.

Hyperledger Cactus Plugins/Connectors Used

• Fabric

Additional context

Mostly the issue is with the collectDefaultMetrics function located under /cactus/packages/cactus-plugin-ledger-connector-fabric/src/main/typescript/prometheus-exporter/prometheus-exporter.ts. Requires more root cause analysis before concluding the reason of the error.

[petermetz]

@jagpreetsinghsasan My recommendation here is to pull the exporter state out of the individual plugins to a dedicated singleton-ish** instance and then have the N number of Fabric connector plugins share that instance among themselves.

Easiest would be to have a PrometheusExporterFabricFactory (or similarly named) factory class that has a static getOrCreate() method that internally checks if there has been any instantiation of the Prometheus exporter and then either creates a new instance of returns the existing one to which the reference can be stored on the global object under a hardcoded uuid as the property key so as to make conflicts practically impossible but also guarantee that different imports of the same factory class will not cause a split brain scenario with multiple singletons even in the same NodeJS process.

There's other options but if you ask me they are all much more heavy handed rabbit-holes, most notably IoC containers, that will still mostly suffer from the same problems anyway due to some specifics of the language/NodeJS itself.

** the ish suffix is there because of the various fine prints when it comes to singletons such as that singleton only in the context of a single NodeJS process, not so much if one uses NodeJS clustering for example.