Set Prometheus SDK Exporter spec to Mixed Stability

**What are you trying to achieve?**

The work here is similar to https://github.com/open-telemetry/opentelemetry-specification/pull/4860. It sets the spec to mixed stability, allowing us to take incremental steps towards stabilization.

Once this task is done, new issues will be opened to stabilize each part of the spec.