[log] Add debug logging to expandTracingVariables in config/expand.go

[github-actions]

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)

Warning

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Generated by Go Logger Enhancement · ● 7.3M · ◷

[Copilot]

Pull request overview

Adds debug logging to internal/config/expand.go’s expandTracingVariables helper so tracing-related ${VAR} expansion is easier to diagnose during config load (particularly the TOML path before validation), leveraging the existing config:validation debug logger used throughout the config package.

Changes:

• Log function entry with presence flags for each tracing field (endpoint, traceId, spanId, headers).
• Log after successful expansion of each non-empty tracing field.
• Log function completion.

Show a summary per file

File	Description
internal/config/expand.go	Adds logValidation debug statements around tracing config variable expansion to improve observability during startup/config load.

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