examples: provide an example for postgres plugin

[mayurkale22]

Provide a full example on how the OpenTelemetry JS library can be used to instrument postgres and export them to Jaeger/Zipkin backend using OpenTelemetry Jaeger/Zipkin exporter.

https://github.com/open-telemetry/opentelemetry-js/tree/master/packages/opentelemetry-plugin-postgres/opentelemetry-plugin-pg

[xiao-lix]

I can take this if it's still available. 👀

[dyladan]

@xiao-lix idk if you have started working on this or not but the example I'm using in my local development of mysql should be fairly easy to swap out for the pg module.

https://github.com/dynatrace-oss-contrib/opentelemetry-js/tree/f99e1743fc1e8f1d9180289aea8da7d8c735b79f/examples/mysql

[dyladan]

if you have not started working, I could easily modify my example to include pg as an option, the same way the examples allow Zipkin or jaeger as an option

[xiao-lix]

@dyladan Hi, I've started working on it and almost done. May have different implementation with your example, please feel free to comment after I create the PR. Thanks. :)