feat: Support for tracing and metrics correlation, as well as more metrics for go runtime information

[dadrus]

This PR includes

• refactoring of code related to metrics collection
• replacement of the previously used middleware for metrics by a custom implementation as it did not really support collecting of metrics endpoints different from the endpoint which exposes the gathered metrics. That brought is a small bug as well, which would not allow gathering of metrics for routes called /metrics processed by heimdall in any of its operation modes.
• correlation of tracing information (if available) with metrics where applicable.
• updates to the documentation related to exported prometheus metrics (mainly the operations/observability section).

[codecov]

Codecov Report

Merging #359 (65a2a93) into main (c366a56) will increase coverage by 0.11%.
The diff coverage is 68.86%.

@@            Coverage Diff             @@
##             main     #359      +/-   ##
==========================================
+ Coverage   86.37%   86.48%   +0.11%     
==========================================
  Files         181      190       +9     
  Lines        7535     7693     +158     
==========================================
+ Hits         6508     6653     +145     
- Misses        872      885      +13     
  Partials      155      155              

Impacted Files	Coverage Δ
internal/handler/decision/module.go	0.00% <0.00%> (-41.82%)	⬇️
internal/handler/management/module.go	0.00% <0.00%> (-41.82%)	⬇️
internal/handler/metrics/app.go	0.00% <0.00%> (ø)
internal/handler/metrics/handler.go	0.00% <0.00%> (ø)
internal/handler/metrics/module.go	0.00% <0.00%> (ø)
internal/handler/proxy/module.go	0.00% <0.00%> (-42.86%)	⬇️
internal/prometheus/module.go	0.00% <0.00%> (ø)
internal/handler/decision/app.go	72.97% <72.97%> (ø)
internal/handler/proxy/app.go	73.68% <73.68%> (ø)
internal/handler/management/app.go	75.00% <75.00%> (ø)
... and 6 more

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