sdk/trace: Log when an attribute was discarded or truncated due to limits

[pellared]

From https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#span-limits:

There SHOULD be a message printed in the SDK's log to indicate to the user that an attribute was discarded due to such a limit. To prevent excessive logging, the message MUST be printed at most once per span (i.e., not per discarded attribute, event, or link).