Attributes API for reading attributes

[HaloFour]

I'm not sure how to quantify this but the API for reading attributes from a ReadableSpan feels a bit weird to me. Like the way that it's a wildcard generic API makes it tricky to use outside of simple scenarios.

For example, I'm toying with a SpanProcessor that can copy the attributes from the parent span to the new span when it is started:

import java.util.Map;
import java.util.Objects;
import java.util.function.Consumer;
import java.util.function.Predicate;

import io.opentelemetry.api.common.AttributeKey;
import io.opentelemetry.api.trace.Span;
import io.opentelemetry.context.Context;
import io.opentelemetry.sdk.trace.ReadWriteSpan;
import io.opentelemetry.sdk.trace.ReadableSpan;
import io.opentelemetry.sdk.trace.SpanProcessor;

public class SpanAttributeInheritingProcessor implements SpanProcessor {
    private final Predicate<String> predicate;

    public SpanAttributeInheritingProcessor() {
        this(key -> true);
    }

    public SpanAttributeInheritingProcessor(Predicate<String> predicate) {
        Objects.requireNonNull(predicate);
        this.predicate = predicate;
    }

    @Override
    public void onStart(Context parentContext, ReadWriteSpan span) {
        var parentSpan = Span.fromContext(parentContext);
        if (parentSpan instanceof ReadableSpan) {
            var readableSpan = (ReadableSpan) parentSpan;
            readableSpan.toSpanData()
                    .getAttributes()
                    .asMap()
                    .entrySet()
                    .stream()
                    .filter(entry -> predicate.test(entry.getKey().getKey()))
                    .forEach(copyAttribute(span));
        }
    }

    private Consumer<Map.Entry<AttributeKey<?>, Object>> copyAttribute(ReadWriteSpan span) {
        return entry -> {
            @SuppressWarnings("unchecked")
            var attributeKey = (AttributeKey<Object>) entry.getKey();
            span.setAttribute(attributeKey, entry.getValue());
        };
    }

    @Override
    public boolean isStartRequired() {
        return true;
    }

    @Override
    public void onEnd(ReadableSpan span) { }

    @Override
    public boolean isEndRequired() {
        return false;
    }
}

I'd be interested in a solution that could avoid suppressing the unchecked warning.

I don't know if this is a use case that would be common enough to justify a new API. However, one that might be helpful could be a method on Span/SpanBuilder called setAttributes that accepts an Attributes instance to be copied onto that Span.

Then the above code could be simplified to the following:

    @Override
    public void onStart(Context parentContext, ReadWriteSpan span) {
        var parentSpan = Span.fromContext(parentContext);
        if (parentSpan instanceof ReadableSpan) {
            var readableSpan = (ReadableSpan) parentSpan;
            var attributes = readableSpan.toSpanData().getAttributes();
            span.setAttributes(attributes);
        }
    }

[jkwatson]

We've discussed extending the Attribute consumer API quite a few times. One option is to provide a non-SMI API with methods to implement for each of the attribute types. Another would be to provide a SMI API that took a whole slew of Consumer implementations, one for each attribute type.

But, yes, it might just be simpler to add a method to the span/builder to set all the Attributes at one go.