[confmap] Add logger to ConverterSettings (open-telemetry#10135)

#### Description
Adds a Logger to ConverterSettings to enable logging from within
converters. Also update the expand converter to log a warning if the env
var is empty or missing.

#### Link to tracking issue
Closes
open-telemetry#9162
Closes
open-telemetry#5615

#### Testing

Unit tests and local testing


![image](https://github.com/open-telemetry/opentelemetry-collector/assets/12352919/af5dd1e2-62f9-4272-97c7-da57166ef07e)

```yaml
receivers:
  nop:

exporters:
  otlphttp:
    endpoint: http://0.0.0.0:4317
    headers:
      # Not set
      x-test: $TEMP3
  debug:
    # set to "detailed"
    verbosity: $TEMP

service:
  telemetry:
    logs:
      level: info
  pipelines:
    traces:
      receivers:
        - nop
      exporters:
        - otlphttp
        - debug
```

#### Documentation
Added godoc comments

.chloggen/confmap-converter-logger.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
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Allow Converters to write logs during startup
+
+# One or more tracking issues or pull requests related to the change
+issues: [10135]
+
+# (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: []

confmap/converter.go

@@ -5,10 +5,18 @@ package confmap // import "go.opentelemetry.io/collector/confmap"
 
 import (
 	"context"
+
+	"go.uber.org/zap"
 )
 
 // ConverterSettings are the settings to initialize a Converter.
-type ConverterSettings struct{}
+type ConverterSettings struct {
+	// Logger is a zap.Logger that will be passed to Converters.
+	// Converters should be able to rely on the Logger being non-nil;
+	// when instantiating a Converter with a ConverterFactory,
+	// nil Logger references should be replaced with a no-op Logger.
+	Logger *zap.Logger
+}
 
 // ConverterFactory defines a factory that can be used to instantiate
 // new instances of a Converter.

confmap/converter/expandconverter/expand.go

@@ -27,10 +27,10 @@ func NewFactory() confmap.ConverterFactory {
 	return confmap.NewConverterFactory(newConverter)
 }
 
-func newConverter(_ confmap.ConverterSettings) confmap.Converter {
+func newConverter(set confmap.ConverterSettings) confmap.Converter {
 	return converter{
 		loggedDeprecations: make(map[string]struct{}),
-		logger:             zap.NewNop(), // TODO: pass logger in ConverterSettings
+		logger:             set.Logger,
 	}
 }
 
@@ -80,6 +80,12 @@ func (c converter) expandEnv(s string) string {
 		if str == "$" {
 			return "$"
 		}
-		return os.Getenv(str)
+		val, exists := os.LookupEnv(str)
+		if !exists {
+			c.logger.Warn("Configuration references unset environment variable", zap.String("name", str))
+		} else if len(val) == 0 {
+			c.logger.Info("Configuration references empty environment variable", zap.String("name", str))
+		}
+		return val
 	})
 }

confmap/converter/expandconverter/expand_test.go

@@ -254,5 +254,5 @@ func TestDeprecatedWarning(t *testing.T) {
 }
 
 func createConverter() confmap.Converter {
-	return NewFactory().Create(confmap.ConverterSettings{})
+	return NewFactory().Create(confmap.ConverterSettings{Logger: zap.NewNop()})
 }

confmap/resolver.go

@@ -94,6 +94,10 @@ func NewResolver(set ResolverSettings) (*Resolver, error) {
 		set.ProviderSettings.Logger = zap.NewNop()
 	}
 
+	if set.ConverterSettings.Logger == nil {
+		set.ConverterSettings.Logger = zap.NewNop()
+	}
+
 	var providers map[string]Provider
 	var converters []Converter
 

confmap/resolver_test.go

@@ -425,6 +425,10 @@ func TestProvidesDefaultLogger(t *testing.T) {
 	_, err := NewResolver(ResolverSettings{
 		URIs:              []string{filepath.Join("testdata", "config.yaml")},
 		ProviderFactories: []ProviderFactory{factory},
+		ConverterFactories: []ConverterFactory{NewConverterFactory(func(set ConverterSettings) Converter {
+			assert.NotNil(t, set.Logger)
+			return &mockConverter{}
+		})},
 	})
 	require.NoError(t, err)
 	require.NotNil(t, provider.logger)

otelcol/collector.go

@@ -123,8 +123,9 @@ func NewCollector(set CollectorSettings) (*Collector, error) {
 	bc := newBufferedCore(zapcore.DebugLevel)
 	cc := &collectorCore{core: bc}
 	options := append([]zap.Option{zap.WithCaller(true)}, set.LoggingOptions...)
-	set.ConfigProviderSettings.ResolverSettings.ProviderSettings = confmap.ProviderSettings{Logger: zap.New(cc, options...)}
-	set.ConfigProviderSettings.ResolverSettings.ConverterSettings = confmap.ConverterSettings{}
+	logger := zap.New(cc, options...)
+	set.ConfigProviderSettings.ResolverSettings.ProviderSettings = confmap.ProviderSettings{Logger: logger}
+	set.ConfigProviderSettings.ResolverSettings.ConverterSettings = confmap.ConverterSettings{Logger: logger}
 
 	if configProvider == nil {
 		configProvider, err = NewConfigProvider(set.ConfigProviderSettings)