Add Environment Variable options to flags managed in exporter-toolkit (#607)

* add env var options to flags managed in exporter-toolkit

exporter_test.go

@@ -4,6 +4,9 @@ import (
 	"reflect"
 	"testing"
 	"time"
+
+	"github.com/alecthomas/kingpin/v2"
+	"github.com/prometheus/exporter-toolkit/web/kingpinflag"
 )
 
 func TestParsePositiveDuration(t *testing.T) {
@@ -103,3 +106,61 @@ func TestParseUnixSocketAddress(t *testing.T) {
 		})
 	}
 }
+
+func TestAddMissingEnvironmentFlags(t *testing.T) {
+	expectedMatches := map[string]string{
+		"non-matching-flag":  "",
+		"web.missing-env":    "MISSING_ENV",
+		"web.has-env":        "HAS_ENV_ALREADY",
+		"web.listen-address": "LISTEN_ADDRESS",
+		"web.config.file":    "CONFIG_FILE",
+	}
+	kingpinflag.AddFlags(kingpin.CommandLine, ":9113")
+	kingpin.Flag("non-matching-flag", "").String()
+	kingpin.Flag("web.missing-env", "").String()
+	kingpin.Flag("web.has-env", "").Envar("HAS_ENV_ALREADY").String()
+	addMissingEnvironmentFlags(kingpin.CommandLine)
+
+	// using Envar() on a flag returned from GetFlag()
+	// adds an additional flag, which is processed correctly
+	// at runtime but means that we need to check for a match
+	// instead of checking the envar of each matching flag name
+	for k, v := range expectedMatches {
+		matched := false
+		for _, f := range kingpin.CommandLine.Model().FlagGroupModel.Flags {
+			if f.Name == k && f.Envar == v {
+				matched = true
+			}
+		}
+		if !matched {
+			t.Errorf("missing %s envar for %s", v, k)
+		}
+	}
+}
+
+func TestConvertFlagToEnvar(t *testing.T) {
+	cases := []struct {
+		input  string
+		output string
+	}{
+		{
+			input:  "dot.separate",
+			output: "DOT_SEPARATE",
+		},
+		{
+			input:  "underscore_separate",
+			output: "UNDERSCORE_SEPARATE",
+		},
+		{
+			input:  "mixed_separate_options",
+			output: "MIXED_SEPARATE_OPTIONS",
+		},
+	}
+
+	for _, c := range cases {
+		res := convertFlagToEnvar(c.input)
+		if res != c.output {
+			t.Errorf("expected %s to resolve to %s but got %s", c.input, c.output, res)
+		}
+	}
+}