Prior to this commit, the `DefaultScheduledTaskObservationConvention`
would fail as it tried to add a `KeyValue` to the observation context
that is `null`. This is rejected by the observation registry and should
be prevented. This happened when registered scheduled methods were
lambdas or part of anonymous classes. Those types do not have a
canonical name and return `null` as a value there.
This commit ensures that for these cases, the default convetion uses a
`"ANONYMOUS"` value as the `"code.namespace"` keyvalue.
Fixes gh-31918
ec5f566ba5