Add `limit_marker_ttl` (`SubjectDeleteMarkerTTL`) for JetStream's per-key TTL / delete-marker support

[ilyaryabikin]

Proposed change

nats.py's nats.js.api.KeyValueConfig as long as nats.js.api.StreamConfig do not expose the JetStream's "limit marker" option required to enable per-key TTL with delete markers. While KeyValueConfig's ttl option sets the expiration for values, without limit_marker_ttl the client cannot receive PURGE events when keys expire.

As a workaround, it is currently possible to declare a kv externally, e.g. using the nats-cli: nats kv add mybucket --ttl=60s --marker-ttl=60s.

Proposed solution: add new field limit_marker_ttl to nats.js.api.KeyValueConfig, then add the same field to nats.js.api.StreamConfig and check if it's handled correctly in nats.js.manager.JetStreamManager.add_stream.

Context:

• nats-py version: 2.11.0
• nats-server version: 2.11.8

References:

• Marker TTLs in Streams: https://docs.nats.io/release-notes/whats_new/whats_new_211?ask=SubjectDeleteMarkerTTL#streams
• Receiving watch events for TTL'd objects: Receive watch events for TTL'd KV objects nats-server#3268
• KeyValueConfig with support of LimitMarkerTTL in Go SDK: https://pkg.go.dev/github.com/nats-io/nats.go/jetstream#KeyValueConfig

Use case

I need to watch not only PUT events in a KV bucket, but also PURGE events when keys are automatically deleted according to the bucket's TTL policy. Without limit_marker_ttl, I cannot achieve feature parity with other SDKs and cannot reliably react to key expirations.

Contribution

No response

[swelborn]

Was a pain to sift thru the endpoints and figure this out, but I managed to do so here as a temporary workaround: NERSC/interactEM#293

Possibly didn't need to send the entire config, but I was getting errors when I only sent the two fields, so I first need to get stream config (I think server doesn't accept partial updates?). If you have limit_marker_ttl, it sets the two fields in the go client: https://github.com/nats-io/nats.go/blob/4ec2f446e4cd829a7be3bf9aa16c43a7fddeaed9/jetstream/kv.go#L645