[Plugin]: Add postgres plugin

[markwolff]

Describe the solution you'd like
Add plugin similar to mongodb plugin (#205) for the node.js postgres driver

Additional context
This can be assigned to me if this is a plugin that this project would like to have.

[mayurkale22]

Thanks for opening the issue.

+1 to add this plugin. The (pg) module seems to have enough downloads (4m/month) on npm to mean that there are a lot users - Substantial use.

@open-telemetry/node-approvers and @open-telemetry/node-maintainers WDYT?

[vmarchaud]

Sure, you can find a opencensus implementation there that i've done at my previous company: https://github.com/keymetrics/pm2-io-apm/blob/master/src/census/plugins/pg.ts