`Resource::get` always cloning the returned `Value`

[paullegranddc]

Issue

I noticed the the opentelemetry_sdk::Resource::get(&self, &Key) -> Value needs to returned an owned Value, and thus always clones it.

The operation is not that expensive when the underlying value is a number, of a static/Arc String. But it does require and allocation for reading values that are owned string, and array values.
This is a bit annoying for instance:

• when writing span processors that need to check the presence of a field on the Resource frequently without storing them.
• when writing span exporters that just need a reference to the data for serialization

Proposal

I think a more flexible API would be to return a reference and modify the signature to Resource::get(&self, &Key) -> &Value.
Although this is a breaking change, existing users of that API would just do get(key).cloned() instead if they really need to have a owned value.

Another option if we don't want to break the API is to keep the old get as before and add a get_ref method.

[scottgerring]

This is waiting on the approved PR behind #3198 to get merged , and a subsequent release!

[cijothomas]

We can do additional get_ref method, however I am not sure of the intended use case yet. Resource is given to processors via a set_resource() method, and it is a one-time call. i.e Resource is not passed with each log/span. If Resource manipulation is not in the hot path, then what is the benefit of get_ref?

Here is an example where Resource fields are retrieved and stored one-time in a processor/exporter.

[scottgerring]

Spoke with @paullegranddc about this; we've got a concrete use case in DataDog/dd-trace-rs:

here:

We transform otel spans to datadog spans. Some attribute values and keys need to be mapped to brand new strings, but some others can be used without copying from the otel string.

So we store strings in dd span as Cow<'a, str>; Since you can borrow Value from span attributes we can borrow them (like [https://github.com/DataDog/dd-trace-rs/blob/b8e95c587a5c41fc9c64e54096025159e1df3050/datadog-opentelemetry-mappings/src/transform/mod.rs#L447[…]7C24](https://github.com/DataDog/dd-trace-rs/blob/b8e95c587a5c41fc9c64e54096025159e1df3050/datadog-opentelemetry-mappings/src/transform/mod.rs#L447[%E2%80%A6]7C24](https://github.com/DataDog/dd-trace-rs/blob/b8e95c587a5c41fc9c64e54096025159e1df3050/datadog-opentelemetry-mappings/src/transform/mod.rs#L447C8-L447C24))), but we always have to copy the resource attributes string, because we can't take a reference into the resource.

Because we don't own the lifecycle of the Resource passed into our span converter we can't really assume that it won't change. It is convenient to hold onto a clone of the resource to read out of it, and it would be nice to have fast access into it.