Fix ClassCastException caused by overly broad type check in Dubbo tracing integration with Spring WebFlux.

[heliang666s]

What is the purpose of the change?

Fix ClassCastException caused by overly broad type check in Dubbo tracing integration with Spring WebFlux.
#15349

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.73%. Comparing base (d6b11cf) to head (a22851a).
Report is 78 commits behind head on 3.3.

Files with missing lines	Patch %	Lines
.../handler/DubboClientTracingObservationHandler.java	0.00%	1 Missing ⚠️
.../handler/DubboServerTracingObservationHandler.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##                3.3   #15371   +/-   ##
=========================================
  Coverage     60.73%   60.73%           
+ Complexity    10920    10918    -2     
=========================================
  Files          1886     1886           
  Lines         86162    86162           
  Branches      12913    12913           
=========================================
+ Hits          52328    52332    +4     
+ Misses        28374    28371    -3     
+ Partials       5460     5459    -1     

Flag	Coverage Δ
integration-tests	33.13% <0.00%> (+0.01%)	⬆️
samples-tests	29.35% <0.00%> (-0.01%)	⬇️
unit-tests	58.86% <0.00%> (+0.01%)	⬆️

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[songxiaosheng]

LGTM