config: add support for parsing env variables in configuration

This replaces the last bit of functionality that was opened in open-telemetry#4826 to
support env variable replacement. Pulled the envprovider.go code from https://github.com/open-telemetry/opentelemetry-collector/blob/main/confmap/provider/envprovider/provider.go

Signed-off-by: Alex Boten <[email protected]>

config/config.go

@@ -6,6 +6,7 @@ package config // import "go.opentelemetry.io/contrib/config"
 import (
 	"context"
 	"errors"
+	"regexp"
 
 	"gopkg.in/yaml.v3"
 
@@ -139,6 +140,20 @@ func WithOpenTelemetryConfiguration(cfg OpenTelemetryConfiguration) Configuratio
 
 // ParseYAML parses a YAML configuration file into an OpenTelemetryConfiguration.
 func ParseYAML(file []byte) (*OpenTelemetryConfiguration, error) {
+	re := regexp.MustCompile(`\$\{([a-zA-Z_][a-zA-Z0-9_]*[-]?.*)\}`)
+
+	replaceEnvVars := func(input []byte) []byte {
+		return re.ReplaceAllFunc(input, func(s []byte) []byte {
+			match := re.FindSubmatch(s)
+			if len(match) < 2 {
+				return s
+			}
+			return replaceEnvVar(string(match[1]))
+		})
+	}
+
+	file = replaceEnvVars(file)
+
 	var cfg OpenTelemetryConfiguration
 	err := yaml.Unmarshal(file, &cfg)
 	if err != nil {

config/config_test.go

@@ -438,6 +438,40 @@ func TestParseYAML(t *testing.T) {
 	}
 }
 
+func TestParseYAMLWithEnvironmentVariables(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		wantErr  error
+		wantType interface{}
+	}{
+		{
+			name:     "valid v0.2 config with env vars",
+			input:    "v0.2-env-var.yaml",
+			wantType: &v02OpenTelemetryConfig,
+		},
+	}
+
+	t.Setenv("OTEL_SDK_DISABLED", "false")
+	t.Setenv("OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT", "4096")
+	t.Setenv("OTEL_EXPORTER_OTLP_PROTOCOL", "http/protobuf")
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			b, err := os.ReadFile(filepath.Join("testdata", tt.input))
+			require.NoError(t, err)
+
+			got, err := ParseYAML(b)
+			if tt.wantErr != nil {
+				require.Equal(t, tt.wantErr.Error(), err.Error())
+			} else {
+				require.NoError(t, err)
+				assert.Equal(t, tt.wantType, got)
+			}
+		})
+	}
+}
+
 func TestSerializeJSON(t *testing.T) {
 	tests := []struct {
 		name     string

config/envprovider.go

@@ -0,0 +1,38 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package config // import "go.opentelemetry.io/contrib/config"
+
+import (
+	"os"
+	"regexp"
+	"strings"
+)
+
+const ValidationPattern = `^[a-zA-Z_][a-zA-Z0-9_]*$`
+
+var validationRegexp = regexp.MustCompile(ValidationPattern)
+
+func replaceEnvVar(uri string) []byte {
+	envVarName, defaultValuePtr := parseEnvVarURI(uri)
+	if !validationRegexp.MatchString(envVarName) {
+		return nil
+	}
+
+	val, exists := os.LookupEnv(envVarName)
+	if !exists {
+		if defaultValuePtr != nil {
+			val = *defaultValuePtr
+		}
+	}
+	return []byte(val)
+}
+
+func parseEnvVarURI(uri string) (string, *string) {
+	const defaultSuffix = ":-"
+	if strings.Contains(uri, defaultSuffix) {
+		parts := strings.SplitN(uri, defaultSuffix, 2)
+		return parts[0], &parts[1]
+	}
+	return uri, nil
+}