Fix span logs and tags while logging with level ERROR

[xbj110825]

While using SpanLogsAppender, we found that while logging with level ERROR, the span log event's value is something like io.opentracing.tag.BooleanTag@7251412f. So i search the behavior of other components(e.g. io.opentracing.contrib.jdbc.JdbcTracingUtils.onError), and found that we should log Tag.ERROR.getKey() instead of Tag.ERROR, and tag error.