consumption_metrics: very old cached values can be sent because of migrations

[koivunej]

Internal slack thread: https://neondb.slack.com/archives/C061CPK7UQL/p1726556769067039?thread_ts=1726493755.104459&cid=C061CPK7UQL

Symptom:

Sent/exported synthetic sizes:

1. synthetic_size=500GB at t=0
2. synthetic_size=1000GB at t=10min
3. synthetic_size=500GB at t=20min

More broadly, what happened:

1. synthetic_size=1000GB was reported from pageserver-12.us-east-2
2. as part of deploy, we migrated tenant shard zero to pageserver-11.us-east-2, no cached value
3. synthetic_size=999GB from pageserver-11.us-east-2
4. (a week passes)
5. synthetic_size=500GB from pageserver-11.us-east-2
6. next deploy, we migrated tenant shard zero back to pageserver-12.us-east-2, cached value was found!
7. synthetic_size=1000GB from pageserver-12.us-east-2 (assumed cached)
8. synthetic_size=500GB from pageserver-12.us-east-2 freshly calculated

How to fix it:

• only use the cached value if it is more fresh than 1h?
• remove the cached value on attachedsingle -> secondary transition?

[koivunej]

Discussed in meeting: Generation number increasing by at most 1 to reuse cached values is the best filter.

No idea right now: how to store generations in the cache? Cache might not be leaked outside, so perhaps that is easy to do.

[jcsp]

@skyzh can you take a look at this one when you're back in the office?

[skyzh]

I think storing generation would make sense, and I would like to have a new format for consumption_metrics.json (I assume nobody else except pageserver is using this json file).

• If root["version"] = "v2", decode using the new format, otherwise, decode using v1
• Use JSON to encode each of the metrics key, i.e.,

  struct NewRawMetric {
  key: MetricsKey,
  event_type: EventType,
  generation: Generation,
  timestamp: u64,
  value: u64,
  }

I'm refactoring the storing/restoring code right now...

[skyzh]

... it seems that for upload_metrics_bucket and upload_metrics_http, we will need to keep the original message format

[skyzh]

otherwise, I think I'll have a PR to migrate to the new format, and another patch to implement the ">= generation => do not report" logic