Remove unnecessary optional so example compiles

[mattmassicotte]

There's an unnecessary optional return here that prevents the example from compiling.

Motivation:

It took me a moment to figure out how to fix this, but in the end it was very straightforward. The prepareMetadata doesn't ever need to return nil.

Modifications:

A small type change fixes the compilation errors in log(event:)

Result:

The example in the documentation can now be used unmodified.