Add type safe Value definition for OTTL

[bogdandrutu]

Updates #45365

This PR adds all supported types and the concept of a "Value". A value can be a concrete type value or a nil value of that type. There is a special "NoneValue" that will be used for functions that don't return anything.

Skipping changelog since this code is not yet plugged to be used in the binary.

[bogdandrutu]

Question: Do we want to support automatic type conversion when possible, or ask users to explicitly call a conversion function like "String(attributes["foo"])"

[florianl]

Maybe just a personal preference:
Instead of implementing NewXValue and NewNilXValue for every type, just have NewXValue and figure out the nil internally before calling newValue().

[iblancasa]

Do we really want to do a full crash here? Also, I feel we should, at least, provide some information about why this happened or something.

[bogdandrutu]

What else should be the behavior? Similar to when you de-reference a pointer, I am not sure what can I do, because if I return a "zero-initialized" value will corrupt/produce wrong results.

[iblancasa]

How about returning an error and drop, somehow, that data?

[bogdandrutu]

Sorry, why is that better? Isn't the same one check caller needs to do?

[iblancasa]

Well, with panic, the collector crashes, leading to all data lost. With the error... maybe one bad record, collector keeps running.
I agree caller should to the check but there can be mistakes. We can fail from the caller but providing some extra context.

At least, something like fmt.Sprintf("ottlruntime: Val() called on nil %T value", v.v)

[bogdandrutu]

Maybe just a personal preference:
Instead of implementing NewXValue and NewNilXValue for every type, just have NewXValue and figure out the nil internally before calling newValue().

@florianl can you help me with a code snippet to understand what is the proposal on how do you know if the value is nil or not?

[florianl]

Maybe just a personal preference:
Instead of implementing NewXValue and NewNilXValue for every type, just have NewXValue and figure out the nil internally before calling newValue().

@florianl can you help me with a code snippet to understand what is the proposal on how do you know if the value is nil or not?

My idea is going in the direction as discussed in #45410 (comment).

The options I see are like this:

func NewBoolValue(val *bool) BoolValue {
	if val == nil {
		return BoolValue{value: newValue(false, true)}
	}
	return BoolValue{value: newValue(*val, false)}
}

But reading your comment, it sounds like that is not a preferred option due to the allocation.

An alternative could look like this:

func NewBoolValue(arg any) (BoolValue, error) {
    switch v := arg.(type) {
    case bool:
        return BoolValue{value: newValue(val, false)}, nil
    case nil:
        return BoolValue{value: newValue(false, true)}, nil
    default:
        return value{}, fmt.Errorf("invalid type")
    }
}