postgres_exporter: updated to 0.5.1

v0.5.1:
* Add application_name as a label for pg_stat_replication metrics

v0.5.0:
It's been far too long since I've done a release and we have a lot of accumulated changes.
* Docker image now runs as a non-root user named "postgres_exporter"
* Add `--auto-discover-databases` option, which automatically discovers and scrapes all databases.
* Add support for boolean data types as metrics
* Replication lag is now expressed as a float and not truncated to an integer.
* When default metrics are disabled, no version metrics are collected anymore either.
* BUGFIX: Fix exporter panic when postgres server goes down.
* Add support for collecting metrics from multiple servers.
* PostgreSQL 11 is now supported in the integration tests.

(adam)