Make context available in log record processor onEmit

[jack-berg]

Related to #2911.

Context is not available to LogRecordProcessor#onEmit implementations at the moment. The API says that when emitting log records you must be able to set Trace Context, not a full Context. Accessing context in LogRecordProcessor#onEmit is important for a variety of use cases, including extracting and baggage entries as attributes on log records.

Implementations have to rely on languages supporting implicit context, something like:

    SdkLoggerProvider.builder()
        .addLogRecordProcessor(
            logRecord -> {
              Baggage baggage = Baggage.fromContext(Context.current());
              baggage
                  .asMap()
                  .forEach(
                      (key, entry) ->
                          logRecord.setAttribute(AttributeKey.stringKey(key), entry.getValue()));
            });

If implicit context is unavailable (i.e the language doesn't support it or the caller set the context explicitly) then accessing context is not possible.

This PR proposes extending onEmit with an additional argument, Context.

An alternative I considered was making Context accessible on ReadWriteLogRecord. This is a worse design because it causes references to Context to have to be held on longer in BatchLogRecordProcessor implementations.

[reyang]

LGTM with a changelog entry.

[tigrannajaryan]

LGTM. Consistent with what we do in SpanProcessor.