[feature] Initial Prometheus metrics implementation (#2334)

* feat: Initial OTEL metrics

* docs: add metrics documentation

* fix: metrics endpoint conditional check

* feat: metrics endpoint basic auth

* fix: make metrics-auth-enabled default false

* fix: go fmt helpers.gen.go

* fix: add metric-related env vars to envparsing.sh

* fix: metrics docs

* fix: metrics related stuff in envparsing.sh

* fix: metrics docs

* chore: metrics docs wording

* fix: metrics stuff in envparsing?

* bump otel versions

---------

Co-authored-by: Tsuribori <user@acertaindebian>
Co-authored-by: Tsuribori <none@example.org>
Co-authored-by: tsmethurst <tobi.smethurst@protonmail.com>

internal/metrics/metrics.go

@@ -0,0 +1,82 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+//go:build !nometrics
+
+package metrics
+
+import (
+	"errors"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/technologize/otel-go-contrib/otelginmetrics"
+	"github.com/uptrace/bun"
+	"github.com/uptrace/bun/extra/bunotel"
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/exporters/prometheus"
+	sdk "go.opentelemetry.io/otel/sdk/metric"
+	"go.opentelemetry.io/otel/sdk/resource"
+	semconv "go.opentelemetry.io/otel/semconv/v1.17.0"
+)
+
+const (
+	serviceName = "GoToSocial"
+)
+
+func Initialize() error {
+	if !config.GetMetricsEnabled() {
+		return nil
+	}
+
+	if config.GetMetricsAuthEnabled() {
+		if config.GetMetricsAuthPassword() == "" || config.GetMetricsAuthUsername() == "" {
+			return errors.New("metrics-auth-username and metrics-auth-password must be set when metrics-auth-enabled is true")
+		}
+	}
+
+	r, _ := resource.Merge(
+		resource.Default(),
+		resource.NewWithAttributes(
+			semconv.SchemaURL,
+			semconv.ServiceName(serviceName),
+		),
+	)
+
+	prometheusExporter, err := prometheus.New()
+	if err != nil {
+		return err
+	}
+
+	meterProvider := sdk.NewMeterProvider(
+		sdk.WithResource(r),
+		sdk.WithReader(prometheusExporter),
+	)
+	otel.SetMeterProvider(meterProvider)
+
+	return nil
+}
+
+func InstrumentGin() gin.HandlerFunc {
+	return otelginmetrics.Middleware(serviceName)
+}
+
+func InstrumentBun() bun.QueryHook {
+	return bunotel.NewQueryHook(
+		bunotel.WithMeterProvider(otel.GetMeterProvider()),
+	)
+}