Support measurement.unit

[marandaneto]

Description

https://develop.sentry.dev/sdk/event-payloads/transaction/

{
  "measurements": {
    "my.custom.metric": { "value": 456, "unit": "millisecond" }
  }
}

MeasurementValue class serialize and deserialize should add support for it, it's currently losing its unit from Hybrid SDKs