Event: Document TTL and best-effort-ness

Generally try to waive away folks who see a particular event stream
and feel tempted to extrapolate and build tooling that expects the
same underlying resource transition chain to continue to produce a
similar event stream as the underlying components evolve and are
updated.  New controllers should not be constrained to be
backwards-compatible with previous versions with regard to Event
emission.  This is distinct from the Event type itself, which has the
usual Kubernetes-API compatibility commitments for versioned types.

The EventTTL default has been 1h since 7e258b85bd (Reduce TTL for
events in etcd from 48hrs to 1hr, 2015-03-11, #5315), and remains so
today:

  $ git --no-pager log -1 --format='%h %s' origin/master
  8e5c02255c Merge pull request #90942 from ii/ii-create-pod%2Bpodstatus-resource-lifecycle-test
  $ git --no-pager grep EventTTL: 8e5c02255c cmd/kube-apiserver/app/options/options.go
  8e5c02255cc:cmd/kube-apiserver/app/options/options.go:		EventTTL:               1 * time.Hour,

In this space [1,2]:

  To avoid filling up master's disk, a retention policy is enforced:
  events are removed one hour after the last occurrence.  To provide
  longer history and aggregation capabilities, a third party solution
  should be installed to capture events.
  ...
  Note: It is not guaranteed that all events happening in a cluster
  will be exported to Stackdriver.  One possible scenario when events
  will not be exported is when event exporter is not running
  (e.g. during restart or upgrade).  In most cases it's fine to use
  events for purposes like setting up metrics and alerts, but you
  should be aware of the potential inaccuracy.
  ...
  To prevent disturbing your workloads, event exporter does not have
  resources set and is in the best effort QOS class, which means that
  it will be the first to be killed in the case of resource
  starvation.

Although that's talking more about export from etcd -> external
storage, and not about cluster components submitting events to etcd.

[1]: https://kubernetes.io/docs/tasks/debug-application-cluster/events-stackdriver/
[2]: https://github.com/kubernetes/website/pull/4155/files#diff-d8eb69c5436aa38b396d4f3ed75e4792R10

pkg/apis/core/types.go

@@ -4602,7 +4602,12 @@ const (
 
 // +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
 
-// Event is a report of an event somewhere in the cluster.
+// Event is a report of an event somewhere in the cluster.  Events
+// have a limited retention time and triggers and messages may evolve
+// with time.  Event consumers should not rely on the timing of an event
+// with a given Reason reflecting a consistent underlying trigger, or the
+// continued existence of events with that Reason.  Events should be
+// treated as informative, best-effort, supplemental data.
 // TODO: Decide whether to store these separately or with the object they apply to.
 type Event struct {
 	metav1.TypeMeta

staging/src/k8s.io/api/core/v1/types.go

@@ -5315,7 +5315,12 @@ const (
 // +genclient
 // +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
 
-// Event is a report of an event somewhere in the cluster.
+// Event is a report of an event somewhere in the cluster.  Events
+// have a limited retention time and triggers and messages may evolve
+// with time.  Event consumers should not rely on the timing of an event
+// with a given Reason reflecting a consistent underlying trigger, or the
+// continued existence of events with that Reason.  Events should be
+// treated as informative, best-effort, supplemental data.
 type Event struct {
 	metav1.TypeMeta `json:",inline"`
 	// Standard object's metadata.