[nexus] Add /v1/ping endpoint

[david-crespo]

Closes #3923

Adds /v1/ping that always returns { "status": "ok" } if it returns anything at all. I went with ping over the initial /v1/system/health because the latter is vague about its meaning, whereas everyone know ping means a trivial request and response. I also thought it was weird to put an endpoint with no auth check under /v1/system, where ~all the other endpoints require fleet-level perms.

This doesn't add too much over hitting an existing endpoint, but I think it's worth it because

• It doesn't hit the DB
• It has no auth check
• It gives a very simple answer to "what endpoint should I use to ping the API?" (a question we have gotten at least once)
• It's easy (I already did it)

Questions that occurred to me while working through this:

• Should we actually attempt to do something in the handler that would tell us, e.g., whether the DB is up?
  • No, that would be more than a ping
  • Raises DoS questions if not auth gated
  • Could add a db status endpoint or or you could use any endpoint that returns data
• What tag should this be under?
  • Initially added a system tag because a) this doesn't fit under existing system/blah tags and b) it really does feel miscellaneous
  • Changed to system/status, with the idea that if we add other kinds of checks, they would be new endpoints under this tag.

[jmpesp]

Should we actually attempt to do something in the handler that would tell us, e.g., whether the DB is up?

Maybe SELECT 1?

[david-crespo]

I think I should change the status key to something more specific (like api) so we can return status info on various things, like DB, existing VMs, provisioning new VMs. We may not yet have operational definitions of "up" for all of those things, so we can add them as we figure that out.

[ahl]

The code looks good, but I'd suggest asking @davepacheco to evaluate the concept.

[davepacheco]

Code looks fine and I appreciate that it's transparent about what it is. This doesn't really solve most of the problems I expect a customer to have in this area but I could see it being useful as a low bar for availability.

[david-crespo]

Yeah, I figure it's a trivially easy nice-to-have. Pretty low value, but the value is still higher than the cost.