chore: modified statsMiddleware to be generic

[saurav-malani]

Description

Modified statsMiddleware to be more generics. So, that we can import and use it at other places as well, like regulation-manager.

Notion Ticket

https://www.notion.so/rudderstacks/make-gateway-middleware-generic-use-it-in-regulation-manager-749a8f97268e46c3bff11d8474d1ce84

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[fracasula]

Looks good but the code is not formatted (linting step is not passing).

[codecov]

Codecov Report

Base: 44.08% // Head: 44.05% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (775eb25) compared to base (f13ea33).
Patch coverage: 42.85% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2626      +/-   ##
==========================================
- Coverage   44.08%   44.05%   -0.04%     
==========================================
  Files         187      187              
  Lines       39255    39255              
==========================================
- Hits        17305    17292      -13     
- Misses      20845    20856      +11     
- Partials     1105     1107       +2     

Impacted Files	Coverage Δ
gateway/gateway.go	55.76% <0.00%> (ø)
middleware/middleware.go	94.82% <100.00%> (ø)
enterprise/reporting/setup.go	38.09% <0.00%> (-14.29%)	⬇️
utils/httputil/server.go	80.76% <0.00%> (-11.54%)	⬇️
enterprise/reporting/reporting.go	8.33% <0.00%> (-1.44%)	⬇️
services/rsources/handler.go	69.72% <0.00%> (-1.39%)	⬇️
config/backend-config/namespace_config.go	73.95% <0.00%> (+3.12%)	⬆️

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