Support an `exception:` param on `Logger#on_emit with semconv attribute bridging

Spec requirement

The API MAY accept an optional Exception parameter (api.md:127-129 (permalink)); if provided, the SDK MUST by default set attributes from the exception per the exception semantic conventions, and user-provided attributes MUST take precedence and MUST NOT be overwritten (sdk.md:233-237 (permalink)); any such attributes MUST follow the OTel exception-attribute semantic conventions (data-model.md:442-443 (permalink)).

Current behavior

No exception: parameter exists on either Logger#on_emit method (API: logs_api/lib/opentelemetry/logs/logger.rb:44-56 (permalink); SDK: logs_sdk/lib/opentelemetry/sdk/logs/logger.rb:66-76 (permalink)) — there is no code path for exception-derived attributes.

Suggested fix

Add an exception: keyword arg to both on_emit methods. In the SDK, before finalizing LogRecord attributes, merge semconv exception.* attributes derived from the exception, without overwriting any attribute the caller already supplied.

Related rows in SPEC_COMPLIANCE_LOGS.md

API-10, SDK-21, DM-5

Start with logs_api/lib/opentelemetry/logs/logger.rb:44-56 and logs_sdk/lib/opentelemetry/sdk/logs/logger.rb:66-76, then compare the referenced API, SDK, and data-model specification sections. Verify the existing logs tests and SPEC_COMPLIANCE_LOGS.md entries API-10, SDK-21, and DM-5. Done means both methods accept exception:, exception attributes follow the semantic conventions, and caller attributes take precedence.