Add reload subcommand

[binarylogic]

We need a snychronous sub-command that reloads configuration. From the systemd docs:

Note however that reloading a daemon by sending a signal (as with the example line above) is usually not a good choice, because this is an asynchronous operation and hence not suitable to order reloads of multiple services against each other. It is strongly recommended to set ExecReload= to a command that not only triggers a configuration reload of the daemon, but also synchronously waits for it to complete.

I assume we can just execute the same path we do when we receive a SIGHUP, but we must wait to respond only after the change is complete.

[lukesteensen]

This might be a good time to introduce the beginning of an admin HTTP API. The subcommand could then just hit a synchronous /reload endpoint.

[CameronNemo]

The asynchronicity is only a problem if you need to do ordering based on the reload, right? What use case did you have in mind?

[sinacek]

Hi, I have use case for similar behaviour vectordotdev/helm-charts#281 (comment) . I'm looking for http endpoint which will reload the configuration in same way, how it does Prometheus.