package main

import (
    "errors"
    "expvar" // New import
    "fmt"
    "net"
    "net/http"
    "strings"
    "sync"
    "time"

    "greenlight.alexedwards.net/internal/data"
    "greenlight.alexedwards.net/internal/validator"

    "golang.org/x/time/rate"
)

...

func (app *application) metrics(next http.Handler) http.Handler {
    // Initialize the new expvar variables when the middleware chain is first built.
    var (
        totalRequestsReceived           = expvar.NewInt("total_requests_received")
        totalResponsesSent              = expvar.NewInt("total_responses_sent")
        totalProcessingTimeMicroseconds = expvar.NewInt("total_processing_time_μs")
    )

    // The following code will be run for every request...
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Record the time that we started to process the request.
        start := time.Now()

        // Use the Add() method to increment the number of requests received by 1.
        totalRequestsReceived.Add(1)

        // Call the next handler in the chain.
        next.ServeHTTP(w, r)

        // On the way back up the middleware chain, increment the number of responses
        // sent by 1.
        totalResponsesSent.Add(1)

        // Calculate the number of microseconds since we began to process the request,
        // then increment the total processing time by this amount.
        duration := time.Since(start).Microseconds()
        totalProcessingTimeMicroseconds.Add(duration)
    })
}

package main

...

func (app *application) routes() http.Handler {
    router := httprouter.New()

    router.NotFound = http.HandlerFunc(app.notFoundResponse)
    router.MethodNotAllowed = http.HandlerFunc(app.methodNotAllowedResponse)

    router.HandlerFunc(http.MethodGet, "/v1/healthcheck", app.healthcheckHandler)

    router.HandlerFunc(http.MethodGet, "/v1/movies", app.requirePermission("movies:read", app.listMoviesHandler))
    router.HandlerFunc(http.MethodPost, "/v1/movies", app.requirePermission("movies:write", app.createMovieHandler))
    router.HandlerFunc(http.MethodGet, "/v1/movies/:id", app.requirePermission("movies:read", app.showMovieHandler))
    router.HandlerFunc(http.MethodPatch, "/v1/movies/:id", app.requirePermission("movies:write", app.updateMovieHandler))
    router.HandlerFunc(http.MethodDelete, "/v1/movies/:id", app.requirePermission("movies:write", app.deleteMovieHandler))

    router.HandlerFunc(http.MethodPost, "/v1/users", app.registerUserHandler)
    router.HandlerFunc(http.MethodPut, "/v1/users/activated", app.activateUserHandler)

    router.HandlerFunc(http.MethodPost, "/v1/tokens/authentication", app.createAuthenticationTokenHandler)

    router.Handler(http.MethodGet, "/debug/vars", expvar.Handler())

    // Use the new metrics() middleware at the start of the chain.
    return app.metrics(app.recoverPanic(app.enableCORS(app.rateLimit(app.authenticate(router)))))
}