feat(discovery): add endpoint for plugins to check their registration

[andrewazores]

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---

Related to cryostatio/cryostat-agent#85

Description of the change:

This adds a GET endpoint that plugins can use with their existing registration ID and token to check their registration status. The API response is 200 if the plugin is registered and the token is still valid, 401 if the token is invalid, and 404 if the token is somehow valid but the plugin is not registered (this should be impossible - if the plugin is not registered then any token it still has should be invalid).

Motivation for the change:

This is used by the -agent to perform its own periodic heartbeat checks on the Cryostat server at a slower cadence. If the plugin notices that it cannot reach Cryostat, or that Cryostat no longer recognizes it as a registered plugin, then the -agent re-enters its original unregistered state and begins trying to more quickly re-register itself with the Cryostat server it is configured for.

How to manually test:

Patch shometest.sh like so:

diff --git a/smoketest.sh b/smoketest.sh
index 4aebf722..6465b53e 100755
--- a/smoketest.sh
+++ b/smoketest.sh
@@ -179,6 +179,7 @@ runDemoApps() {
         --env CRYOSTAT_AGENT_TRUST_ALL="true" \
         --env CRYOSTAT_AGENT_AUTHORIZATION="Basic $(echo user:pass | base64)" \
         --env CRYOSTAT_AGENT_REGISTRATION_PREFER_JMX="true" \
+        --env CRYOSTAT_AGENT_REGISTRATION_CHECK_MS="15000" \
         --rm -d quay.io/andrewazores/quarkus-test:latest
 
     # copy a jboss-client.jar into /clientlib first

to make one of the sample agent applications perform this check more frequently so it's easier to catch.

1. Run CRYOSTAT_IMAGE=quay.io... sh smoketest.sh...
2. Check the server logs and agent logs. There should be frequent GET requests to the new endpoint, which should succeed.
3. TODO figure out a decent way to set up an environment where the Cryostat server can be taken down and restarted without also taking down the agents.

[github-actions]

Test image available:

$ CRYOSTAT_IMAGE=ghcr.io/cryostatio/cryostat:pr-1425-0f7f509e7b1590cc62c774f2e06ed99d219ac4cc sh smoketest.sh

[maxcao13]

Looks good to me!

[github-actions]

Test image available:

$ CRYOSTAT_IMAGE=ghcr.io/cryostatio/cryostat:pr-1425-66ae507d9f6e0d8745bbee8d97e6a070e07c058a sh smoketest.sh