logging: clarify error logging

contributors/devel/sig-instrumentation/logging.md

@@ -60,8 +60,29 @@ Good practices when writing log messages are:
 
 ### What method to use?
 
-* `klog.ErrorS` - Errors should be used to indicate unexpected behaviours in code, like unexpected errors returned by subroutine function calls.
-Logs generated by `ErrorS` command may be enhanced with additional debug information (by logging library). Calling `ErrorS` with `nil` as error may be acceptable if there is error condition that deserves a stack trace at this origin point.
+* `klog.ErrorS`, [`logr.Logger.Error`](https://pkg.go.dev/github.com/go-logr/logr#Logger.Error):
+  Use error logs to inform admins that they might have to do something to fix a problem.
+  If there is no `error` instance, pass nil and use the message string and key/value pairs to describe the problem instead of fabricating an error with `fmt.Errorf`.
+  If the dependency is allowed, use `runtime.HandleErrorWithContext` or `runtime.HandleErrorWithLogger` instead.
+
+  Don't emit an error log before returning an error because
+  it is usually uncertain if and how the caller is going to handle the returned error.
+  It might handle it gracefully, in which case alerting an administrator with an error log would be wrong.
+  Instead, use `fmt.Errorf` with `%w` to return a more informative error with a trail for where the error came from.
+  Avoid "failed to" as prefix when wrapping errors, it quickly becomes repetitive when added multiple times.
+
+  Sometimes it may be useful to log an error for debugging purposes before returning it. Use an info log with at least
+  `V(4)` and `err` as key for the key/value pair.
+
+* [`runtime.HandleErrorWithContext`](https://pkg.go.dev/k8s.io/[email protected]/pkg/util/runtime#HandleErrorWithContext),
+  [`runtime.HandleErrorWithLogger`](https://pkg.go.dev/k8s.io/[email protected]/pkg/util/runtime#HandleErrorWithLogger):
+  Whenever possible, use this instead of normal error logging at the top of a call chain, i.e. the place where code
+  cannot return an error up to its caller, if there is no other way of handling the error (like logging it
+  and then exiting the process).
+
+  The `runtime` package supports additional error handling mechanisms that downstream consumers of Kubernetes may add
+  (see [ErrorHandlers](https://pkg.go.dev/k8s.io/[email protected]/pkg/util/runtime#pkg-variables)).
+  Some [downstream consumers](https://grep.app/search?q=runtime.ErrorHandlers) even suppress logging of errors completely.
 
 * `klog.InfoS` -  Structured logs to the INFO log. `InfoS` should be used for routine logging. It can also be used to log warnings for expected errors (errors that can happen during routine operations).
   Depending on log severity it's important to pick a proper verbosity level to ensure that consumer is neither under nor overwhelmed by log volume: