How to register Micrometer Timer with SLA and tags?

Question

I'm trying to migrate my Prometheus metrics to micrometer but now I'm stuck with one thing here...

At the moment I have a Prometheus histogram configured as follows:

private static final Histogram REQUEST_DURATION = Histogram
        .build("http_request_duration_milliseconds", "Duration in milliseconds for processing a request.")
        .labelNames("http_method", "http_status", "java_class", "java_method")
        .buckets(10, 25, 50, 100, 500, 1000)
        .register();

So for switching to Micrometer I replaced it as follows:

Timer.builder("http.request.duration")
            .description("Duration in seconds for processing a request.")
            .sla(Duration.ofMillis(10), Duration.ofMillis(25), Duration.ofMillis(50), Duration.ofMillis(100), Duration.ofMillis(500), Duration.ofMillis(1000), Duration.ofMillis(5000))
            .register(registry);

Ok. Let's see how I want to use it... At the moment I simply call

REQUEST_DURATION.labels(httpMethod, httpStatus, javaClass, javaMethod).observe(milliseconds);

So I replaced this to

Metrics.timer("http.request.duration",
            "http.method", httpMethod,
            "http.status", httpStatus,
            "java.class", javaClass,
            "java.method", javaMethod)
            .record(Duration.ofNanos(nanoseconds));

But the problem now is, that Micrometer complains that I previously configured the metric without those tags. Of course I did, because I don't know the values at that point. Here the Exception:

java.lang.IllegalArgumentException: Prometheus requires that all meters with the same name have the same set of tag keys. There is already an existing meter containing tag keys []. The meter you are attempting to register has keys [http.method, http.status, java.class, java.method].

Ok. So I thought, then let's specify the buckets with the Metrics.timer call. But that doesn't work because there is no method for passing these values.

So... How can I set the sla buckets and the tags for my metric?

Answer 1

I got the answer on Micrometer slack channel. The Micrometer-way of solving this, is not to register the metric itself but instead to register a filter as follows:

registry.config().meterFilter(new MeterFilter() {
    @Override
    public DistributionStatisticConfig configure(Meter.Id id, DistributionStatisticConfig config) {
        if (id.getName().equals("http.request.duration")) {
            return DistributionStatisticConfig.builder()
                    .sla(Duration.ofMillis(10).toNanos(),
                         Duration.ofMillis(25).toNanos(),
                         Duration.ofMillis(50).toNanos(), 
                         Duration.ofMillis(100).toNanos(),
                         Duration.ofMillis(500).toNanos(),
                         Duration.ofMillis(1000).toNanos(), 
                         Duration.ofMillis(5000).toNanos())
                    .build()
                    .merge(config);
        }
        return config;
    }
});

When pushing a metric value using Metrics.timer(...) as above Micrometer will call this filter and apply all the configuration specified here. This filter is only called on meter initialization, i.e. when Metrics.timer(...) is called the first time with this specific name and tags. So we don't have to worry about performance here.