Since `/api/v1/write` is a mutating endpoint, we should still activate
the remote-write-receiver explicitly. But we should do it in the same
way as the other mutating endpoints, i.e. via a flag
`--web.enable-remote-write-receiver`.
This commit marks the feature flag as deprecated, i.e. it still works
but logs a warning on startup. This enables users to seamlessly
migrate. With the next minor release, we can start ignoring the
feature flag (but still warn a user that is trying to use it).
Signed-off-by: beorn7 <beorn@grafana.com>
8fdfa52976