Metrics Annotations

Frameworks like Spring Boot, MicroProfile, and Dropwizard offer annotations like @Timed or @Counted for creating metrics from method calls.

The benefit is that developers can provide metrics for their business logic while still separating business code from metrics tracking.

@Timed
@Counted
public void processPayment() {
    // business code here
}

The OpenTelemetry Java auto instrumentation supports the @WithSpan annotation for tracing, but has nothing equivalent for metrics yet.

Brainstorming of a few ideas:

• For @Timed the type of histogram should be configurable, like @Timed(histogramType = EXPONENTIAL). Other histogram types would be explicit buckets and default buckets.
• Explicit buckets should be configurable: @Timed(buckets = { 0.001, 0.002, 0.003, 0.004 })
• For exponential histograms the resolution should be configurable.
• Static attributes should be configurable: @Counted(attributes = { "key", "value" })
• Dynamic attributes should be configurable similar to OpenTelemetry's @SpanAttribute.
• Dynamic attributes based on the return value and on an exception thrown would be great.

Moreover, it would be awesome to integrate this with @WithSpan so that develpers can say "I want to time this method AND have a Span". However, it should also be possible to use tracing without metrics, because histograms may introduce a lot of cardinality, and users might just want to create a Span without creating a histogram.

What do you think, is it worthwhile to put some more thoughts into this?

No implementation files, tests, or entry points are named. Start by reviewing the existing @WithSpan auto-instrumentation and the discussion of @Timed and @Counted requirements. The work is not ready for a newcomer until the annotation API, supported metric options, tracing interaction, and tests defining done are agreed.