Return HandlerChain from Routes()

[superatrain]

This PR adds HandlersChain []HandlerFunc to the RouteInfo object returned from Engine.Routes(). This will return the handlers, in order, associated with each route.

Unlike context.HandlerNames(), this can be called for all routes, and can be used at service startup, during unit tests, etc. Our service at work applies different middleware to different routes dynamically, and my intention is to use this to catch unintended changes to the chain.

Inspired by #1131 and #1729

Sample usage:

type RouteDetail struct {
	Method   string   `json:"method"`
	Path     string   `json:"path"`
	Handlers []string `json:"handlers"`
}
routeDetails := make([]RouteDetail, 0)
for _, route := range engine.Routes() {
	handlers := []string{}
	for _, handler := range route.HandlersChain {
		handlers = append(handlers, runtime.FuncForPC(reflect.ValueOf(handler).Pointer()).Name())
	}
	routeDetails = append(routeDetails, RouteDetail{
		Method:   route.Method,
		Path:     route.Path,
		Handlers: handlers,
	})
}
file, _ := json.MarshalIndent(routeDetails, "", " ")
_ = ioutil.WriteFile("routes.json", file, 0644)

[codecov]

Codecov Report

Merging #2518 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2518   +/-   ##
=======================================
  Coverage   98.45%   98.46%           
=======================================
  Files          41       41           
  Lines        1948     1949    +1     
=======================================
+ Hits         1918     1919    +1     
  Misses         17       17           
  Partials       13       13           

Impacted Files	Coverage Δ
gin.go	99.01% <ø> (+<0.01%)	⬆️

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