fix: set request ID in tracing context key (#17602)

Since `addCustomerHeaders` middleware was after the `httpTracer`
middleware, the request ID was not set in the http tracing context. By
reordering these middleware functions, the request ID header becomes
available. We also avoid setting the tracing context key again in
`newContext`.

Bonus: All middleware functions are renamed with a "Middleware" suffix
to avoid confusion with http Handler functions.

cmd/http-tracer.go

@@ -65,7 +65,7 @@ func getOpName(name string) (op string) {
 
 // If trace is enabled, execute the request if it is traced by other handlers
 // otherwise, generate a trace event with request information but no response.
-func httpTracer(h http.Handler) http.Handler {
+func httpTracerMiddleware(h http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		// Setup a http request response recorder - this is needed for
 		// http stats requests and audit if enabled.