Update to v0.9.1

Changelogs:

• https://github.com/prometheus/pushgateway/releases/tag/v0.9.1
• https://github.com/prometheus/pushgateway/releases/tag/v0.9.0
• https://github.com/prometheus/pushgateway/releases/tag/v0.7.0
• https://github.com/prometheus/pushgateway/releases/tag/v0.5.2
• https://github.com/prometheus/pushgateway/releases/tag/v0.5.1
• https://github.com/prometheus/pushgateway/releases/tag/v0.5.0
• https://github.com/prometheus/pushgateway/releases/tag/v0.4.0

The new persistent storage format is not backwards compatible. See the
changelogs for details.

• Define LICENSE_FILE.
• Switch to USES=go:modules
• Simplify targets.
• Fix a couple of typos and style issues in the service file
• Improve readability of comments in the service file
• Do not install /var/db/pushgateway by default. It is going to be created by the service anyway.

PR: 233660
Approved by: maintainer
Event: EuroBSDcon 2019 Hackathon
Differential Revision: https://reviews.freebsd.org/D21729