Update sslmode for postgres-exporter

Summary

A customer is seeing level=info msg="Error opening connection to database (user=gitlab-psql%20host=0.0.0.0%20database=postgres%20sslmode=allow): pq: unsupported sslmode \"allow\"; only \"require\" (default), \"verify-full\", \"verify-ca\", and \"disable\" supported" source="postgres_exporter.go:1012"

We are currently setting sslmode=allow by default.

It does not appear that a sslmode of allow is supported in the Go PostgreSQL driver.

Version 10.6.4 to 10.7.4.

Proposed fix

Change sslmode=allow to sslmode=require. The driver defaults sslmode to require.

Link

Customer ticket -> https://gitlab.zendesk.com/agent/tickets/97417 (internal)

Workaround

See comment #3570 (comment 189998863)