[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>

vendor/github.com/prometheus/client_golang/prometheus/labels.go

@@ -0,0 +1,186 @@
+// Copyright 2018 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package prometheus
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"unicode/utf8"
+
+	"github.com/prometheus/common/model"
+)
+
+// Labels represents a collection of label name -> value mappings. This type is
+// commonly used with the With(Labels) and GetMetricWith(Labels) methods of
+// metric vector Collectors, e.g.:
+//
+//	myVec.With(Labels{"code": "404", "method": "GET"}).Add(42)
+//
+// The other use-case is the specification of constant label pairs in Opts or to
+// create a Desc.
+type Labels map[string]string
+
+// LabelConstraint normalizes label values.
+type LabelConstraint func(string) string
+
+// ConstrainedLabels represents a label name and its constrain function
+// to normalize label values. This type is commonly used when constructing
+// metric vector Collectors.
+type ConstrainedLabel struct {
+	Name       string
+	Constraint LabelConstraint
+}
+
+// ConstrainableLabels is an interface that allows creating of labels that can
+// be optionally constrained.
+//
+//	prometheus.V2().NewCounterVec(CounterVecOpts{
+//	  CounterOpts: {...}, // Usual CounterOpts fields
+//	  VariableLabels: []ConstrainedLabels{
+//	    {Name: "A"},
+//	    {Name: "B", Constraint: func(v string) string { ... }},
+//	  },
+//	})
+type ConstrainableLabels interface {
+	compile() *compiledLabels
+	labelNames() []string
+}
+
+// ConstrainedLabels represents a collection of label name -> constrain function
+// to normalize label values. This type is commonly used when constructing
+// metric vector Collectors.
+type ConstrainedLabels []ConstrainedLabel
+
+func (cls ConstrainedLabels) compile() *compiledLabels {
+	compiled := &compiledLabels{
+		names:            make([]string, len(cls)),
+		labelConstraints: map[string]LabelConstraint{},
+	}
+
+	for i, label := range cls {
+		compiled.names[i] = label.Name
+		if label.Constraint != nil {
+			compiled.labelConstraints[label.Name] = label.Constraint
+		}
+	}
+
+	return compiled
+}
+
+func (cls ConstrainedLabels) labelNames() []string {
+	names := make([]string, len(cls))
+	for i, label := range cls {
+		names[i] = label.Name
+	}
+	return names
+}
+
+// UnconstrainedLabels represents collection of label without any constraint on
+// their value. Thus, it is simply a collection of label names.
+//
+//	UnconstrainedLabels([]string{ "A", "B" })
+//
+// is equivalent to
+//
+//	ConstrainedLabels {
+//	  { Name: "A" },
+//	  { Name: "B" },
+//	}
+type UnconstrainedLabels []string
+
+func (uls UnconstrainedLabels) compile() *compiledLabels {
+	return &compiledLabels{
+		names: uls,
+	}
+}
+
+func (uls UnconstrainedLabels) labelNames() []string {
+	return uls
+}
+
+type compiledLabels struct {
+	names            []string
+	labelConstraints map[string]LabelConstraint
+}
+
+func (cls *compiledLabels) compile() *compiledLabels {
+	return cls
+}
+
+func (cls *compiledLabels) labelNames() []string {
+	return cls.names
+}
+
+func (cls *compiledLabels) constrain(labelName, value string) string {
+	if fn, ok := cls.labelConstraints[labelName]; ok && fn != nil {
+		return fn(value)
+	}
+	return value
+}
+
+// reservedLabelPrefix is a prefix which is not legal in user-supplied
+// label names.
+const reservedLabelPrefix = "__"
+
+var errInconsistentCardinality = errors.New("inconsistent label cardinality")
+
+func makeInconsistentCardinalityError(fqName string, labels, labelValues []string) error {
+	return fmt.Errorf(
+		"%w: %q has %d variable labels named %q but %d values %q were provided",
+		errInconsistentCardinality, fqName,
+		len(labels), labels,
+		len(labelValues), labelValues,
+	)
+}
+
+func validateValuesInLabels(labels Labels, expectedNumberOfValues int) error {
+	if len(labels) != expectedNumberOfValues {
+		return fmt.Errorf(
+			"%w: expected %d label values but got %d in %#v",
+			errInconsistentCardinality, expectedNumberOfValues,
+			len(labels), labels,
+		)
+	}
+
+	for name, val := range labels {
+		if !utf8.ValidString(val) {
+			return fmt.Errorf("label %s: value %q is not valid UTF-8", name, val)
+		}
+	}
+
+	return nil
+}
+
+func validateLabelValues(vals []string, expectedNumberOfValues int) error {
+	if len(vals) != expectedNumberOfValues {
+		return fmt.Errorf(
+			"%w: expected %d label values but got %d in %#v",
+			errInconsistentCardinality, expectedNumberOfValues,
+			len(vals), vals,
+		)
+	}
+
+	for _, val := range vals {
+		if !utf8.ValidString(val) {
+			return fmt.Errorf("label value %q is not valid UTF-8", val)
+		}
+	}
+
+	return nil
+}
+
+func checkLabelName(l string) bool {
+	return model.LabelName(l).IsValid() && !strings.HasPrefix(l, reservedLabelPrefix)
+}