Set pgsql exporter round from db on init

[Eric-Warehime]

Summary

Same change that was made for the noop exporter in #1176

We want to set the exporter's round based on the expected state so that the pipeline knows which round to start on.

Test Plan

Dummy Db always returns round 0, so there's not a great way to unit test this built in currently.

[codecov]

Codecov Report

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