"unsigned.age" field missing from events

This issue was originally created by @p-e-w at https://github.com/matrix-org/dendrite/issues/1754.

Background information

Dendrite version or git SHA: 0.3.9
Monolith or Polylith?: Monolith
SQLite3 or Postgres?: Postgres
Running in Docker?: No
go version: 1.15

Description

Synapse -> Synapse

{
  ...
  "origin_server_ts": 1612587703370,
  "unsigned": {
    "age": 4060
  }
}

Dendrite -> Synapse

The age field is missing:

{
  ...
  "origin_server_ts": 1612588027946,
  "unsigned": {}
}

Synapse -> Dendrite

A non-canonical age_ts field is present, which appears to always be equal to origin_server_ts:

{
  ...
  "origin_server_ts": 1612588032002,
  "unsigned": {
    "age_ts": 1612588032002
  }
}

Why this matters

The entire unsigned field is explicitly marked as optional by the spec, but the age subfield is useful for breaking down latency measurements into more fine-grained metrics. This is done by PingPong, which currently cannot break down latencies when Dendrite is involved because of this issue.

element-hq / dendrite