experiment: prototype measurement processor

[martintmk]

In this PR I am trying to experiment with dynamic metric enrichment. This API tries to do the enrichment in a naive way and should not affect he performance if this feature is not used.

I have experimented with using Cow<'a, [KeyValue]> as an argument to the processor. The processor might decide to not modify the attributes and, in that case, the original slice is returned without any additional allocations.

When measurement processors are not used, I have not detected any changes in throughput.

Fixes #
Design discussion issue (if applicable) #

Changes

Please provide a brief description of the changes here.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[cijothomas]

could the trait be modified to accept slice of attributes, and return Option ? If processor has no interest in updating, it can return None, at which point the caller uses the original slice itself.
If processor wants to change, it'll create a new Vec! and return it.
Cost is paid only when one wants to enrich..

[martintmk]

I have updated the trait with this suggestion. Looks nicer indeed.

[martintmk]

FYI: One big advantage of using Cow is the re-use of owned Vec in case multiple processors are modifying the attributes:

impl MeasurementProcessor for UserTypeMeasurementProcessor {
    fn process<'a>(&self, attributes: Cow<'a, [KeyValue]>) -> Cow<'a, [KeyValue]> {
        match Context::current().get::<UserType>() {
            Some(user_type) => {
                let mut attrs = attributes.into_owned();
                attrs.push(KeyValue::new("user_type", user_type.as_str()));
                Cow::Owned(attrs)
            }

            // No changes to the attributes
            None => attributes,
        }
    }
}

The into_owned will either:

• Return already allocated vector: Cow::Owned
• Create a new vector in case the Cow contained the borrowed data.