Copy LICENSE and NOTICE into the Docker image, matching what the
release tarballs already ship and the prometheus/prometheus Dockerfile
convention.
Fixes#1206
Signed-off-by: Dean Chen <862469039@qq.com>
Revamp the build system to be more inline with other Prometheus exporters.
Notably add Darwin and Windows build targets, and add support for releases
using tar files.
This is based on the mysqld_exporter, but drastically changes how metrics are
discovered and enumerated since postgres already very closely follows the
Prometheus format.
