Consistent type to express date & time

[svrnm]

While looking at different semantic conventions I saw that different notations for time are used, i.e. UNIX timestamps as int and formatted date as strings:

semantic-conventions/docs/attributes-registry/messaging.md

Line 117 in a41f0e8

| `messaging.rocketmq.message.delivery_timestamp` | int | The timestamp in milliseconds that the delay message is expected to be delivered to consumer. | `1665987217045` |

semantic-conventions/docs/resource/cloud-provider/heroku.md

Line 14 in a41f0e8

| `heroku.release.creation_timestamp` | string | Time and date the release was created | `2022-10-23T18:00:42Z` | Opt-In |

semantic-conventions/docs/messaging/rocketmq.md

Line 25 in a41f0e8

| [`messaging.rocketmq.message.delivery_timestamp`](../attributes-registry/messaging.md) | int | The timestamp in milliseconds that the delay message is expected to be delivered to consumer. | `1665987217045` | Conditionally Required: [2] |

semantic-conventions/docs/attributes-registry/tls.md

Line 19 in a41f0e8

| `tls.client.not_after` | string | Date/Time indicating when client certificate is no longer considered valid. | `2021-01-01T00:00:00.000Z` |

I think there should be one consistent way to express those OR at least a _timestamp should be equivalent to a unix timestamp and a _datetime should be a formatted string

[pyohannes]

Good point. ECS supports different representations for date & time, but at least has it clearly documented: https://www.elastic.co/guide/en/elasticsearch/reference/8.12/date.html

[svrnm]

via #564 (comment) (@jsuereth)

We had some in person discussions in the Specification meeting and the Semconv meeting.
I'm not sure we have consensus here, but I do think we can likely make progress by sticking (consistently) to String-dates in ISO 8601 format in semconv (what you have).

I am wondering where this should be added, from what I see there is no file that describes attribute values in more details, beyond what is in https://github.com/open-telemetry/opentelemetry-specification/tree/main/specification/common?