[log] Add debug logging to expandTracingVariables in config/expand.go (#5138)

## Summary

Adds meaningful debug logging calls to the `expandTracingVariables`
function in `internal/config/expand.go`.

## Changes

The `expandTracingVariables` function previously had no log calls
despite performing non-trivial variable expansion across multiple
tracing config fields. The rest of `config/expand.go` and its sibling
`config/validation.go` already use the `logValidation` logger
extensively.

**New logging added:**
- Function entry log showing which tracing config fields are present
(`hasEndpoint`, `hasTraceID`, `hasSpanID`, `hasHeaders`)
- Individual expansion log per field (endpoint, traceId, spanId,
headers) — only emitted when that field is non-empty
- Function completion log

## Why This File

- The `logValidation` logger (`config:validation` namespace) is already
declared in the same package (`config/validation.go`), so no new logger
declaration is needed
- `expandTracingVariables` is called for TOML-loaded configs during
startup — logging here helps diagnose tracing misconfiguration where
`\$\{VAR}` syntax is used in OpenTelemetry settings
- The 5 new log calls are proportionate and non-redundant; each message
adds distinct value

## Validation

- `go build` ✅
- `go vet` ✅  
- `go test ./...` ✅ (pre-existing `TestFetchAndFixSchema_NetworkError`
network failure unrelated to this change)




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Author: lpcox

internal/config/expand.go

@@ -97,11 +97,15 @@ func expandTracingVariables(cfg *TracingConfig) error {
 		return nil
 	}
 
+	logValidation.Printf("Expanding tracing config variables: hasEndpoint=%v, hasTraceID=%v, hasSpanID=%v, hasHeaders=%v",
+		cfg.Endpoint != "", cfg.TraceID != "", cfg.SpanID != "", cfg.Headers != "")
+
 	if cfg.Endpoint != "" {
 		expanded, err := expandVariables(cfg.Endpoint, "gateway.opentelemetry.endpoint")
 		if err != nil {
 			return err
 		}
+		logValidation.Printf("Expanded tracing endpoint variable")
 		cfg.Endpoint = expanded
 	}
 
@@ -110,6 +114,7 @@ func expandTracingVariables(cfg *TracingConfig) error {
 		if err != nil {
 			return err
 		}
+		logValidation.Printf("Expanded tracing traceId variable")
 		cfg.TraceID = expanded
 	}
 
@@ -118,6 +123,7 @@ func expandTracingVariables(cfg *TracingConfig) error {
 		if err != nil {
 			return err
 		}
+		logValidation.Printf("Expanded tracing spanId variable")
 		cfg.SpanID = expanded
 	}
 
@@ -126,8 +132,10 @@ func expandTracingVariables(cfg *TracingConfig) error {
 		if err != nil {
 			return err
 		}
+		logValidation.Printf("Expanded tracing headers variable")
 		cfg.Headers = expanded
 	}
 
+	logValidation.Print("Tracing config variable expansion completed")
 	return nil
 }