Add support for metricExpression in SLOs

[angrykirc]

Is your feature request related to a problem? Please describe. Yes

Describe the solution you'd like Add metricExpression support to SloService.create and to the Slo class.

Describe alternatives you've considered There is no alternative solutions found yet

Additional context Since metricDenominator, metricNumerator and metricRate are deprecated, to create a new SLO, a metricExpression field is required. The existing API does not pass this field to the body of the JSON request.

[Dynatrace-James-Kitson]

Thank you for raising this, it sounds like the API had some changes and we'll need to update this endpoint.