I am trying to use and extend spring boot’s Observation API solution for instrumenting metrics for spring mvc http.server.requests.

I’m trying to add a custom tag, with the key duration, and a value of http request execution time in ms. For this I have provided a @Bean that extends DefaultServerRequestObservationConvention from the org.springframework.http.server.observation package like so:

public class ExtendedServerRequestObservationConvention extends DefaultServerRequestObservationConvention {

    ...

    @Override
    @NonNull
    public KeyValues getHighCardinalityKeyValues(@NonNull ServerRequestObservationContext context) {
        return super.getHighCardinalityKeyValues(context).and(addTag(context));
    }

    // additional tag that we want to include in the metrics
    protected KeyValue addTag(ServerRequestObservationContext context) {
        String requestDuration = context.get("duration");
        assert requestDuration != null;
        return KeyValue.of("duration", requestDuration);
    }
}```

To measure the duration I’ve tried using the Timer from the Observation.Context and adding the value to the context with context.put(), like so:

public class ExtendedObservationHandler implements ObservationHandler<ServerRequestObservationContext> {

    ...
    @Override
    public void onStop(ServerRequestObservationContext context) {
        LongTaskTimer.Sample sample = context.get(LongTaskTimer.Sample.class);
        if (sample != null) {
            // Get the duration in the desired time unit, e.g., milliseconds
            double duration = sample.duration(java.util.concurrent.TimeUnit.MILLISECONDS);
            String durationStr = String.valueOf(duration);
            System.out.println("Request duration: " + durationStr + "ms");
            context.put("duration", durationStr);
        }

        ObservationHandler.super.onStop(context);
    }

    @Override
    public boolean supportsContext(@NonNull Observation.Context context) {
        return context instanceof ServerRequestObservationContext;
    }
}

The problem is that the duration value is not available when I try to obtain it with context.get('duration') in the ExtendedServerRequestObservationConvention. I have both classes configured as @Bean, they both work, but not ‘together’.

What am I missing? Can someone maybe help? Thank you!