Remove usage of deprecated handler

[epels]

In cmd/sachet/main.go, the deprecated prometheus.Handler() is used.

See: https://godoc.org/github.com/prometheus/client_golang/prometheus#Handler

The docs suggest using promhttp.Handler instead, but that seems to behave slightly different from prometheus.Handler.

Some related discussions in other repos:

• https://github.com/kubernetes/kubernetes/issues/48297
• https://github.com/matrix-org/dendrite/issues/243