add Prometheus percentage metrics for httpserver & proxy

[LokiWager]

Add Prometheus percentage metrics for HTTPServer & Proxy

• add 3 metrics for HTTPServer
  • httpserver_requests_duration_percentage
  • httpserver_requests_size_bytes_percentage
  • httpserver_responses_size_bytes_percentage
• add 2 metrics for Proxy
  • proxy_request_body_size_percentage
  • proxy_response_body_size_percentage

[localvar]

what's the usage of this map?
as floating point numbers often can not be represented precisely, we must be cautious about using them as keys of maps.

[LokiWager]

Objectives is Summary Bucket, which type is map[float64]float64.

I forgot to use the default Objectives here. Please, review it again.

[codecov-commenter]

Codecov Report

Base: 76.23% // Head: 75.80% // Decreases project coverage by -0.42% ⚠️

Coverage data is based on head (20f8049) compared to base (f99cd41).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #879      +/-   ##
==========================================
- Coverage   76.23%   75.80%   -0.43%     
==========================================
  Files         110      114       +4     
  Lines       12867    13387     +520     
==========================================
+ Hits         9809    10148     +339     
- Misses       2510     2663     +153     
- Partials      548      576      +28     

Impacted Files	Coverage Δ
pkg/filters/proxy/pool.go	82.68% <100.00%> (+0.70%)	⬆️
pkg/object/httpserver/mux.go	80.88% <100.00%> (+0.19%)	⬆️
pkg/object/httpserver/runtime.go	68.50% <100.00%> (+2.16%)	⬆️
pkg/object/mqttproxy/topic.go	47.05% <0.00%> (-52.95%)	⬇️
pkg/object/mqttproxy/session.go	67.70% <0.00%> (-14.51%)	⬇️
pkg/object/mqttproxy/client.go	77.45% <0.00%> (-4.16%)	⬇️
pkg/object/mqttproxy/broker.go	73.75% <0.00%> (-1.05%)	⬇️
pkg/cluster/cluster.go	59.57% <0.00%> (-0.53%)	⬇️
pkg/object/mqttproxy/spec.go	100.00% <0.00%> (ø)
pkg/object/mqttproxy/mqttproxy.go	46.83% <0.00%> (ø)
... and 6 more

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