Add support for configurable target header for the request_id middleware

[luminoso]

This PR adds the ability to choose what header to target for the X-Request-ID. Before this PR, the header used by the middleware was hardcoded for X-Request-ID, and exposing it via configuration will allow, among others, to set for X-Correlation-ID.

Setting a different header key is helpful for distributed traceability since X-Request-ID is used to trace single requests. In contrast, X-Correlation-ID(or others) is commonly used to trace multiple servers' transactions.

The default is set as the previous hardcoded header (X-Request-ID), which is a backward-compatible change.

Example for the new configuration:

(...)

rid = RequestIDWithConfig(RequestIDConfig{
		TargetHeader: echo.HeaderXCorrelationID,
})

(...)

This way they can be combined and the same middleware can be used for both use-cases.

Also updated documentation: labstack/echox#239

[codecov]

Codecov Report

Merging #2040 (6dbf4d1) into master (b437ee3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2040   +/-   ##
=======================================
  Coverage   91.32%   91.33%           
=======================================
  Files          33       33           
  Lines        2871     2873    +2     
=======================================
+ Hits         2622     2624    +2     
  Misses        159      159           
  Partials       90       90           

Impacted Files	Coverage Δ
echo.go	94.15% <ø> (ø)
middleware/request_id.go	85.71% <100.00%> (+1.50%)	⬆️

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[aldas]

LGTM.

sidenote: I would assume that if you are adding X-Correlation-ID you want to add X-Request-ID also. These headers are complementary to each other. Anyway this is ok, as you can always add other header with

	e.Use(RequestIDWithConfig(RequestIDConfig{
		RequestIDHandler: func(c echo.Context, s string) {
			corrID := c.Request().Header.Get(echo.HeaderXCorrelationID)
			if corrID != "" {
				c.Response().Header().Set(echo.HeaderXCorrelationID, corrID)
			}
		},
	}))

and there are probably cases when you have different header than X-Request-ID (whatever legacy or 3rdpary reasons) and would like to use middleware with your own custom header.