OTEL propagation through pub/sub

[NathanBaulch]

I'd like to see code that responds to a pub/sub subscription message captured in the OTEL span of the publisher. Is this a solved problem or do I need to roll something myself?
I've been tinkering with a redis.PubSub wrapper that appends the OTEL state (traceparent and tracestate) to the pubsub payload and extracts it out again when a channel message is received. It works, but I'm just wondering if there's an existing solution out there.

[NathanBaulch]

FWIW, this is what I ended up with. https://gist.github.com/NathanBaulch/cf585100454b0afd724ccb6eb70eb334
It would have been nice to hide this behind a hook/middleware but there is no subscription stream hook AFAIK and no way to attach/return context with pub/sub messages anyway.

Notes:

• No access to internal.AppendArg - workaround with redis.NewCmd(ctx, val).String().
• Expose Publish* commands on PubSub instead of redis.UniversalClient since the payload must be instrumented.
• Payload instrumented with prefixed \n delimited OTEL fields to avoid complex encoding/decoding.
• Passing context.Context in a message feels wrong so passing SpanContext instead.
• Can't extract SpanContext - workaround with temporary context trace.SpanContextFromContext(tc.Extract(tmp, mc)).

Example usage:

ps := redisotel.Subscribe(ctx, rds, "foo")
defer ps.Close()

go func() {
	for m := range ps.Channel() {
		ctx := m.WithContext(ctx)
		handleMessage(ctx, m.Payload)
	}
}()

for i := 0; i < 10; i++ {
	ps.Publish(ctx, "foo", i)
}

time.Sleep(time.Second)