feat: add configurable OTEL span processor to enable reformat and reduce noise

[thoriqzs25]

Context

OTEL traces export verbose span names and noisy transport spans that clutter observability backends and increase costs:

• gRPC spans use full paths like /pkg.Service/V0GetStats instead of just V0GetStats
• HTTP transport spans (ServeHTTP) appear in every trace but provide little value

Proposal

Add a configurable span processor layer between span creation and export that supports:

1. Span filtering — drop spans by name or custom criteria before export
2. Span transformation — rename spans (e.g., extract method name from gRPC path)
3. Extensibility — allow services to provide custom filter/transform logic at runtime

Configuration should be available via:

• Environment variables for common cases (filter list, naming format)

Impact

• Reduced trace volume → lower observability costs
• Cleaner span names → easier trace navigation
• Extensible → services can add custom filtering without forking
• No breaking changes — defaults preserve existing behavior

Files

• otel/spanprocessor.go — new SpanProcessor, interfaces, renamedSpan wrapper
• otel/spanprocessor_test.go — unit tests
• config/config.go — new config fields
• initializers.go — wrap BatchSpanProcessor, expose runtime APIs
• core.go — pass config to OTLPConfig

[thoriqzs25]

"ServeHTTP" traces could be considered a noise for projects that are looking for a way to see potential issue within the internal usecases. It would be good to have a way to filter out these traces and focus on internal spans.