Support default values and erorrs for env var expansion (#10907)

#### Description
Support shell-style default value and error message for env var
expansion.
```
// A default value for unset variable can be provided after :- suffix, for example:
// `env:NAME_OF_ENVIRONMENT_VARIABLE:-default_value`
```

#### Link to tracking issue
Fixes #5228

#### Testing
Unit tests

#### Documentation
* [x] Provider Go docs 
* [ ] Probably needs some other docs changes?

---------

Signed-off-by: Yuri Shkuro <github@ysh.us>

.chloggen/envprovider-default-value.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: confmap/provider/envprovider
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Support default values when env var is empty
+
+# One or more tracking issues or pull requests related to the change
+issues: [5228]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: ['user']

confmap/provider/envprovider/provider.go

@@ -27,6 +27,11 @@ type provider struct {
 //
 // This Provider supports "env" scheme, and can be called with a selector:
 // `env:NAME_OF_ENVIRONMENT_VARIABLE`
+//
+// A default value for unset variable can be provided after :- suffix, for example:
+// `env:NAME_OF_ENVIRONMENT_VARIABLE:-default_value`
+//
+// See also: https://opentelemetry.io/docs/specs/otel/configuration/file-configuration/#environment-variable-substitution
 func NewFactory() confmap.ProviderFactory {
 	return confmap.NewProviderFactory(newProvider)
 }
@@ -41,14 +46,18 @@ func (emp *provider) Retrieve(_ context.Context, uri string, _ confmap.WatcherFu
 	if !strings.HasPrefix(uri, schemeName+":") {
 		return nil, fmt.Errorf("%q uri is not supported by %q provider", uri, schemeName)
 	}
-	envVarName := uri[len(schemeName)+1:]
+	envVarName, defaultValuePtr := parseEnvVarURI(uri[len(schemeName)+1:])
 	if !envvar.ValidationRegexp.MatchString(envVarName) {
 		return nil, fmt.Errorf("environment variable %q has invalid name: must match regex %s", envVarName, envvar.ValidationPattern)
-
 	}
+
 	val, exists := os.LookupEnv(envVarName)
 	if !exists {
-		emp.logger.Warn("Configuration references unset environment variable", zap.String("name", envVarName))
+		if defaultValuePtr != nil {
+			val = *defaultValuePtr
+		} else {
+			emp.logger.Warn("Configuration references unset environment variable", zap.String("name", envVarName))
+		}
 	} else if len(val) == 0 {
 		emp.logger.Info("Configuration references empty environment variable", zap.String("name", envVarName))
 	}
@@ -63,3 +72,13 @@ func (*provider) Scheme() string {
 func (*provider) Shutdown(context.Context) error {
 	return nil
 }
+
+// returns (var name, default value)
+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
+}

confmap/provider/envprovider/provider_test.go

@@ -154,6 +154,42 @@ func TestEmptyEnvWithLoggerWarn(t *testing.T) {
 	assert.Equal(t, envName, logLine.Context[0].String)
 }
 
+func TestEnvWithDefaultValue(t *testing.T) {
+	env := createProvider()
+	tests := []struct {
+		name        string
+		unset       bool
+		value       string
+		uri         string
+		expectedVal string
+		expectedErr string
+	}{
+		{name: "unset", unset: true, uri: "env:MY_VAR:-default % value", expectedVal: "default % value"},
+		{name: "unset2", unset: true, uri: "env:MY_VAR:-", expectedVal: ""}, // empty default still applies
+		{name: "empty", value: "", uri: "env:MY_VAR:-foo", expectedVal: ""},
+		{name: "not empty", value: "value", uri: "env:MY_VAR:-", expectedVal: "value"},
+		{name: "syntax1", unset: true, uri: "env:-MY_VAR", expectedErr: "invalid name"},
+		{name: "syntax2", unset: true, uri: "env:MY_VAR:-test:-test", expectedVal: "test:-test"},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if !test.unset {
+				t.Setenv("MY_VAR", test.value)
+			}
+			ret, err := env.Retrieve(context.Background(), test.uri, nil)
+			if test.expectedErr != "" {
+				require.ErrorContains(t, err, test.expectedErr)
+				return
+			}
+			require.NoError(t, err)
+			str, err := ret.AsString()
+			require.NoError(t, err)
+			assert.Equal(t, test.expectedVal, str)
+		})
+	}
+	assert.NoError(t, env.Shutdown(context.Background()))
+}
+
 func createProvider() confmap.Provider {
 	return NewFactory().Create(confmaptest.NewNopProviderSettings())
 }