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, 1c59763, 3829320, f724e82, e189226)

[bsiedleckiallegro]

That is a pretty significant improvement, good job!
@pkanal @JamieDanielson could you please take a look at the changes suggested by @RigoTamas ? Much appreciated :)

[allegro-dbargowski]

Hey there, just wanted to share that I've tried out the changes from this PR by tweaking the OTEL libraries in node_modules for our app, and it sorted out the issues we were facing with http_route values in our Prometheus metrics. @pkanal @JamieDanielson @raphael-theriault-swi , it would be awesome if you could take a look at this PR soon. Thanks a bunch!

[raphael-theriault-swi]

Left some feedback but I don't feel it's important enough to warrant not approving. Thanks for the contribution !

[raphael-theriault-swi]

This is extremely minor but I can't really think of any other information that could be returned by this, so personally I'd rather see this skip the object and just return a boolean, the meaning of which can be documented in the docstring.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.48%. Comparing base (b520d04) to head (f724e82).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2682      +/-   ##
==========================================
- Coverage   91.53%   91.48%   -0.05%     
==========================================
  Files         171      171              
  Lines        8172     8174       +2     
  Branches     1660     1660              
==========================================
- Hits         7480     7478       -2     
- Misses        692      696       +4     

Files with missing lines	Coverage Δ
...try-instrumentation-express/src/instrumentation.ts	98.61% <100.00%> (ø)
...opentelemetry-instrumentation-express/src/utils.ts	98.50% <100.00%> (+0.04%)	⬆️

... and 1 file with indirect coverage changes

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[RigoTamas]

Thanks for approving @raphael-theriault-swi! Who is part of the open-telemetry/javascript-approvers group? I see somebody from that group still needs to approve in order to merge this.

[raphael-theriault-swi]

@open-telemetry/javascript-approvers ready for workflow approval

[allegro-dbargowski]

Hey @raphael-theriault-swi, @pkanal, and @JamieDanielson! Just checking in to see if there's any chance this fix will be merged soon. We're using OTEL for monitoring and alerting based on Prometheus metrics for our app, which is critical for our on-call engineers. Right now, we're seeing issues with invalid http_route label values (which in turn makes differentiating endpoints impossible sometimes). If the merge will take a while, we'll consider using a forked version with this fix applied. Any updates would be greatly appreciated—thanks a bunch! 🙏