Maybe consider to use the same pattern as StartOption from StartSpan in trace.

type metadatas struct {
	ttl int
}

type Metadata func(*metadatas)

// Not exported for the moment because we want only TTLNoPropagation and TTLUnlimitedPropagation
func withTTL(ttl int) Metadata {
  return func(*metadatas) {
    
  }
}

var (
  TTLNoPropagation = withTTL(valueTTLNoPropagation)
  TTLUnlimitedPropagation = withTTL(valueTTLUnlimitedPropagation)
}

func Upsert(k Key, v string, mds ...Metadata) Mutator {
    metas metadatas
    for _, md := range mds {
        op(&metas)
    }
}

Users will use this:

tag.Upsert(key, "my_value", TTLNoPropagation, other_metadata)

What do you think? @rakyll do you have any suggestion?

Why pointer to metadata?

metadata may or may not exists or given kv pair. Also metadata could be reused for several kv pair, In that case it can be reused.

UpsertWithMetadata is a very verbose name for a core API.

Please consider short names. If not, user will never think about UpsertWithMetadata is a replacement for Upsert.

Not sure if we want to expose these constants here. See the comment at census-instrumentation/opencensus-java#1768 (comment).

The intention was to restrict from using any value other than two TTL values. But this is not the right approach as TTLNoPropation can be modified by the user. Let me look at how to do this differently.

Curious why this needs to be func?

sort of like a builder (similar to startOption for startSpan).

What do you think about this?

type TTL struct {
  value int
}

var (
NoPropagation = &TTL{value= 0}
UnlimitedPropagation = &TTL{value= -1}
)

func WithTTL(TTL ttl) Metadata {
	return func(m *metadatas) {
		m.ttl = ttl
	}
}
     

you could inadvertently modify UnlimitedPropagation and WithTTL would allow any value of TTL. I thought we don't want that at this point

@rakyll would you be worried about the possibility of someone by mistake setting UnlimitedPropagation to nil?

Let's do it this way.

@bogdandrutu I have already made the change and uploaded. If everything looks fine then can you please approve it?