In #477 we introduce a new extension for tracking (request receipt) timings,
and reflecting those back through response headers. This comes with some code
that handles configuration. This configuration handling is pretty similar across the
extensions, and there are multiple enhancement opportunities there:

• First it would be nice to factor out a common base for configuration handling
• Second, once we've done that, we can probably generalize testing of configuration handling,
  creating a TEST_P which verifies all extensions are well behaved there.