[output spec] missing timestamp for single measurements

[mimir-d]

context: measurement series elements provision a timestamp field in order to express the fact that the measurement was taken at a different time than the artifact output timestamp (ref: https://github.com/opencomputeproject/ocp-diag-core/blob/main/json_spec/output/measurement_series_element.json#L20)

However, single measurements don't have this (ref: https://github.com/opencomputeproject/ocp-diag-core/blob/main/json_spec/README.md#measurement). The same scenario can arise for these, where the measurement was taken at a different time than the message emission time. It should be added