net-mgmt/pushgateway: Update to 0.10.0

This release changes the storage format. v0.10 can read the storage format
of v0.5–v0.9. It will then persist the new format so that a downgrade
won't be possible anymore[1].

[1] https://github.com/prometheus/pushgateway/releases/tag/v0.10.0

For this port 0.10.0, it is intended to replace it with a version >= 1.3.x
from about four weeks on. So there will be four weeks for the migration.

PR: 250587
Submitted by: Gabriel Dutra <0xdutra@gmail.com> (new maintainer)
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D27165