feat: add Prometheus metrics (#439)

* feat: add Prometheus metrics

* setup metrics using Prometheus client under /metrics endpoint
* add configuration value for metrics
* configure Prometheus middleware for generic metrics
* provide metrics for totals of users, gists and SSH keys
* modify test request to optionally return the response
* provide integration test for Prometheus metrics
* update documentation

* chore: make fmt

internal/web/test/metrics_test.go

@@ -0,0 +1,113 @@
+package test
+
+import (
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/thomiceli/opengist/internal/db"
+	"io"
+	"net/http"
+	"os"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+var (
+	SSHKey = db.SSHKeyDTO{
+		Title:   "Test SSH Key",
+		Content: `ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAklOUpkDHrfHY17SbrmTIpNLTGK9Tjom/BWDSUGPl+nafzlHDTYW7hdI4yZ5ew18JH4JW9jbhUFrviQzM7xlELEVf4h9lFX5QVkbPppSwg0cda3Pbv7kOdJ/MTyBlWXFCR+HAo3FXRitBqxiX1nKhXpHAZsMciLq8V6RjsNAQwdsdMFvSlVK/7XAt3FaoJoAsncM1Q9x5+3V0Ww68/eIFmb1zuUFljQJKprrX88XypNDvjYNby6vw/Pb0rwert/EnmZ+AW4OZPnTPI89ZPmVMLuayrD2cE86Z/il8b+gw3r3+1nKatmIkjn2so1d01QraTlMqVSsbxNrRFi9wrf+M7Q== admin@admin.local`,
+	}
+	AdminUser = db.UserDTO{
+		Username: "admin",
+		Password: "admin",
+	}
+
+	SimpleGist = db.GistDTO{
+		Title:       "Simple Test Gist",
+		Description: "A simple gist for testing",
+		VisibilityDTO: db.VisibilityDTO{
+			Private: 0,
+		},
+		Name:    []string{"file1.txt"},
+		Content: []string{"This is the content of file1"},
+		Topics:  "",
+	}
+)
+
+// TestMetrics tests the metrics endpoint functionality of the application.
+// It verifies that the metrics endpoint correctly reports counts for:
+// - Total number of users
+// - Total number of gists
+// - Total number of SSH keys
+//
+// The test follows these steps:
+// 1. Enables metrics via environment variable
+// 2. Sets up test environment
+// 3. Registers and logs in an admin user
+// 4. Creates a gist and adds an SSH key
+// 5. Queries the metrics endpoint
+// 6. Verifies the reported metrics match expected values
+//
+// Environment variables:
+//   - OG_METRICS_ENABLED: Set to "true" for this test
+func TestMetrics(t *testing.T) {
+	originalValue := os.Getenv("OG_METRICS_ENABLED")
+
+	os.Setenv("OG_METRICS_ENABLED", "true")
+
+	defer os.Setenv("OG_METRICS_ENABLED", originalValue)
+
+	s := Setup(t)
+	defer Teardown(t, s)
+
+	register(t, s, AdminUser)
+	login(t, s, AdminUser)
+
+	err := s.Request("GET", "/all", nil, 200)
+	require.NoError(t, err)
+
+	err = s.Request("POST", "/", SimpleGist, 302)
+	require.NoError(t, err)
+
+	err = s.Request("POST", "/settings/ssh-keys", SSHKey, 302)
+	require.NoError(t, err)
+
+	var metricsRes http.Response
+	err = s.Request("GET", "/metrics", nil, 200, &metricsRes)
+	require.NoError(t, err)
+
+	body, err := io.ReadAll(metricsRes.Body)
+	defer metricsRes.Body.Close()
+	require.NoError(t, err)
+
+	lines := strings.Split(string(body), "\n")
+	var usersTotal float64
+	var gistsTotal float64
+	var sshKeysTotal float64
+
+	for _, line := range lines {
+		if strings.HasPrefix(line, "opengist_users_total") {
+			parts := strings.Fields(line)
+			if len(parts) == 2 {
+				usersTotal, err = strconv.ParseFloat(parts[1], 64)
+				assert.NoError(t, err)
+			}
+		} else if strings.HasPrefix(line, "opengist_gists_total") {
+			parts := strings.Fields(line)
+			if len(parts) == 2 {
+				gistsTotal, err = strconv.ParseFloat(parts[1], 64)
+				assert.NoError(t, err)
+			}
+		} else if strings.HasPrefix(line, "opengist_ssh_keys_total") {
+			parts := strings.Fields(line)
+			if len(parts) == 2 {
+				sshKeysTotal, err = strconv.ParseFloat(parts[1], 64)
+				assert.NoError(t, err)
+			}
+		}
+	}
+
+	assert.Equal(t, 1.0, usersTotal, "opengist_users_total should be 1")
+	assert.Equal(t, 1.0, gistsTotal, "opengist_gists_total should be 1")
+	assert.Equal(t, 1.0, sshKeysTotal, "opengist_ssh_keys_total should be 1")
+}