sysutils/snmp_exporter: update to 0.29.0

new upstream release, changes:
https://github.com/prometheus/snmp_exporter/releases/tag/v0.29.0

while here, update my email address.

Reviewed by: arrowd, kevans
Approved by: arrowd (ports), kevans (mentor)
Pull Request: https://github.com/freebsd/freebsd-ports/pull/381