fix(express): span name if middleware on nested router is used

[RigoTamas]

Which problem is this PR solving?

• Fixes: @opentelemetry/instrumentation-express - incorrect route set to span, if middlewares are being used #2680

Short description of the changes

• A flag is passed down in the storeLayerPath function, which tells the caller wether a new item has been added to the layer store. This is needed, because we only want to pop from the req[_LAYERS_STORE_PROPERTY] list if we added something to it. Popping from this list in case we did not push anything to it will result in popping a parent entry, which will skew the layers store. Hence this unnecessary popping will result in a malformed http.route tag.
• This can be reproduced by adding a middleware to a nested router.
• The nested router unit test has been updated to reflect this behavior. The unit test now breaks without this change, which reflects the need for this change.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: RigoTamas (a49e9d7, e189226)