Add support for custom validations in promlint

Signed-off-by: João Vilaça <jvilaca@redhat.com>

prometheus/testutil/promlint/promlint.go

@@ -35,6 +35,8 @@ type Linter struct {
 	// of them.
 	r   io.Reader
 	mfs []*dto.MetricFamily
+
+	customValidations []validations.Validation
 }
 
 // New creates a new Linter that reads an input stream of Prometheus metrics in
@@ -53,6 +55,14 @@ func NewWithMetricFamilies(mfs []*dto.MetricFamily) *Linter {
 	}
 }
 
+// AddCustomValidations adds custom validations to the linter.
+func (l *Linter) AddCustomValidations(vs ...validations.Validation) {
+	if l.customValidations == nil {
+		l.customValidations = make([]validations.Validation, 0, len(vs))
+	}
+	l.customValidations = append(l.customValidations, vs...)
+}
+
 // Lint performs a linting pass, returning a slice of Problems indicating any
 // issues found in the metrics stream. The slice is sorted by metric name
 // and issue description.
@@ -72,11 +82,11 @@ func (l *Linter) Lint() ([]validations.Problem, error) {
 				return nil, err
 			}
 
-			problems = append(problems, lint(mf)...)
+			problems = append(problems, l.lint(mf)...)
 		}
 	}
 	for _, mf := range l.mfs {
-		problems = append(problems, lint(mf)...)
+		problems = append(problems, l.lint(mf)...)
 	}
 
 	// Ensure deterministic output.
@@ -91,12 +101,19 @@ func (l *Linter) Lint() ([]validations.Problem, error) {
 }
 
 // lint is the entry point for linting a single metric.
-func lint(mf *dto.MetricFamily) []validations.Problem {
+func (l *Linter) lint(mf *dto.MetricFamily) []validations.Problem {
 	var problems []validations.Problem
+
 	for _, fn := range validations.DefaultValidations {
 		problems = append(problems, fn(mf)...)
 	}
 
+	if l.customValidations != nil {
+		for _, fn := range l.customValidations {
+			problems = append(problems, fn(mf)...)
+		}
+	}
+
 	// TODO(mdlayher): lint rules for specific metrics types.
 	return problems
 }

prometheus/testutil/promlint/promlint_test.go

@@ -19,6 +19,8 @@ import (
 	"strings"
 	"testing"
 
+	dto "github.com/prometheus/client_model/go"
+
 	"github.com/prometheus/client_golang/prometheus/testutil/promlint"
 	"github.com/prometheus/client_golang/prometheus/testutil/promlint/validations"
 )
@@ -787,3 +789,60 @@ func runTests(t *testing.T, tests []test) {
 		})
 	}
 }
+
+func TestCustomValidations(t *testing.T) {
+	lintAndVerify := func(l *promlint.Linter, cv test) {
+		problems, err := l.Lint()
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		if want, got := cv.problems, problems; !reflect.DeepEqual(want, got) {
+			t.Fatalf("unexpected problems:\n- want: %v\n-  got: %v",
+				want, got)
+		}
+	}
+
+	prob := []validations.Problem{
+		{
+			Metric: "mc_something_total",
+			Text:   "expected metric name to start with 'memcached_'",
+		},
+	}
+
+	cv := test{
+		name: "metric without necessary prefix",
+		in: `
+# HELP mc_something_total Test metric.
+# TYPE mc_something_total counter
+mc_something_total 10
+`,
+		problems: nil,
+	}
+
+	prefixValidation := func(mf *dto.MetricFamily) []validations.Problem {
+		if !strings.HasPrefix(mf.GetName(), "memcached_") {
+			return []validations.Problem{
+				{
+					Metric: mf.GetName(),
+					Text:   "expected metric name to start with 'memcached_'",
+				},
+			}
+		}
+		return []validations.Problem{}
+	}
+
+	t.Helper()
+	t.Run(cv.name, func(t *testing.T) {
+		// no problems
+		l1 := promlint.New(strings.NewReader(cv.in))
+		lintAndVerify(l1, cv)
+	})
+	t.Run(cv.name, func(t *testing.T) {
+		// prefix problems
+		l2 := promlint.New(strings.NewReader(cv.in))
+		l2.AddCustomValidations(prefixValidation)
+		cv.problems = prob
+		lintAndVerify(l2, cv)
+	})
+}