spanlogger Log should call Error if log level is error

[cstyan]

We use SpanLogger to unify logging and tracing instrumentation code, but we need to call level.Error and log.Error to log and set the error tag on the span in the case of errors. We can keep the current Log and Error functions to satisfy the interfaces but extend Log (or introduce a new function) to detect level=error in the key value pairs passed to the function to also call Error and save users a loc per error handling branch

See discussion starting here: #2970 (comment)

cc @pstibrany @pracucci