Support middleware injected by OpenTelemetry Express Instrumentation plugin

[a-legrand]

OpenTelemetry Express Instrumentation injects a proxy object into the router stack, which it uses for its tracing. This is similar to how NewRelic adds a sentinel handler to the router stack. This commit adds a similar workaround so that loopback can find the original layer.

I made the variable more generic to just test if shimmer is used to wrap the handler, as this library can be used by other libraries.

See also #4119

Checklist

• [x] New tests added or existing tests modified to cover all changes
  • Unit tests have been added
• [x] Code conforms with the style guide
  • All lint tests passes

[adambusfy123]

Whats the status here?