Stricter filtering of check run completion events (#2520)

I observed that 100% of canceled jobs in my runner pool were not causing scale down events. This PR fixes that.

The problem was caused by #2119. 

#2119 ignores certain webhook events in order to fix #2118. However, #2119 overdoes it and filters out valid job cancellation events. This PR uses stricter filtering and add visibility for future troubleshooting.

<details><summary>Example cancellation event</summary>

This is the redacted top portion of a valid cancellation event my runner pool received and ignored.

```json
{
  "action": "completed",
  "workflow_job": {
    "id": 12848997134,
    "run_id": 4738060033,
    "workflow_name": "slack-notifier",
    "head_branch": "auto-update/slack-notifier-0.5.1",
    "run_url": "https://api.github.com/repos/nuru/<redacted>/actions/runs/4738060033",
    "run_attempt": 1,
    "node_id": "CR_kwDOB8Xtbc8AAAAC_dwjDg",
    "head_sha": "55bada8f3d0d3e12a510a1bf34d0c3e169b65f89",
    "url": "https://api.github.com/repos/nuru/<redacted>/actions/jobs/12848997134",
    "html_url": "https://github.com/nuru/<redacted>/actions/runs/4738060033/jobs/8411515430",
    "status": "completed",
    "conclusion": "cancelled",
    "created_at": "2023-04-19T00:03:12Z",
    "started_at": "2023-04-19T00:03:42Z",
    "completed_at": "2023-04-19T00:03:42Z",
    "name": "build (arm64)",
    "steps": [

    ],
    "check_run_url": "https://api.github.com/repos/nuru/<redacted>/check-runs/12848997134",
    "labels": [
      "self-hosted",
      "arm64"
    ],
    "runner_id": 0,
    "runner_name": "",
    "runner_group_id": 0,
    "runner_group_name": ""
  },
```

</details>

controllers/actions.summerwind.net/horizontal_runner_autoscaler_webhook.go

@@ -210,7 +210,16 @@ func (autoscaler *HorizontalRunnerAutoscalerGitHubWebhook) Handle(w http.Respons
 			if e.GetAction() == "queued" {
 				target.Amount = 1
 				break
-			} else if e.GetAction() == "completed" && e.GetWorkflowJob().GetConclusion() != "skipped" && e.GetWorkflowJob().GetRunnerID() > 0 {
+			} else if e.GetAction() == "completed" && e.GetWorkflowJob().GetConclusion() != "skipped" {
+				// We want to filter out "completed" events sent by check runs.
+				// See https://github.com/actions/actions-runner-controller/issues/2118
+				// and https://github.com/actions/actions-runner-controller/pull/2119
+				// But canceled events have runner_id == 0 and GetRunnerID() returns 0 when RunnerID == nil,
+				// so we need to be more specific in filtering out the check runs.
+				// See example check run completion at https://gist.github.com/nathanklick/268fea6496a4d7b14cecb2999747ef84
+				if e.GetWorkflowJob().GetConclusion() == "success" && e.GetWorkflowJob().RunnerID == nil {
+					log.V(1).Info("Ignoring workflow_job event because it does not relate to a self-hosted runner")
+				} else {
 					// A negative amount is processed in the tryScale func as a scale-down request,
 					// that erases the oldest CapacityReservation with the same amount.
 					// If the first CapacityReservation was with Replicas=1, this negative scale target erases that,
@@ -218,6 +227,7 @@ func (autoscaler *HorizontalRunnerAutoscalerGitHubWebhook) Handle(w http.Respons
 					target.Amount = -1
 					break
 				}
+			}
 			// If the conclusion is "skipped", we will ignore it and fallthrough to the default case.
 			fallthrough
 		default: