feat: add spanId and traceSampled logic

[0xSage]

Blocked: Nodejs-logging release

Changes:

• Users can now hardcode a spanId via LOGGING_SPAN_KEY when using Winston
• Middleware scenario: if span is detected from the request obj, it is propagated to makeChildLogger, and emitRequestLog capabilities, and span is included in the final child logs.
• export function getCurrentTraceFromAgent

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[0xSage]

blocked by: https://github.com/googleapis/nodejs-logging/pull/1100

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