Parse basic logging severity text values

[damemi]

Fixes #442

[punya]

The nested boolean logic is a bit hard for me to follow. Would you consider instead:

func cloudLoggingSeverity(log plog.LogRecord) logging.Severity {
  severityNumber := log.SeverityNumber()
  if severityNumber == 0 {
    severityNumber := otelSeverityTexts[strings.ToLower(log.SeverityText())]
  }
  return severityMapping[severityNumber]
}

The double negative in the comment is also difficult, could you rephrase it? e.g., document how severity mapping works overall, rather than this particular sub-case.

[damemi]

updated, I changed the variable declaration and added more comments but kept the nested boolean. Hoping the new comment explains it better, we want to check that the SeverityText exists in our map (in other words, severityNumber == 0 isn't enough on its own to do this logic)