Add async/callback mode to instruments? about prometheus-cpp

[ganli]

I have same requirement, to implement sth like GaugeAdapter to collect metric from our internal statistics data on demand.
It's too complex & inefficient to sync such data with prometheus-cpp's internal registry.
For example, we have one counter to track the number of objects. Internally we use std::atomic_int to count, but it's so inefficient to call Gauge::Set in object's constructor/destructor.

[ganli]

Finally I implemented simple customized registry to do this job:

typedef std::function<double()> GaugeValueGetter;

class PassiveMetricsRegistry: public prometheus::Collectable
{
public:
    bool registerGaugeSource(
        std::string_view measureName,
        std::string_view help,
        const Labels& labels,
        GaugeValueGetter valueGetter);

    bool deregisterGaugeSource(
        std::string_view measureName,
        const Labels& labels);

    std::vector<prometheus::MetricFamily> Collect() const override;
};

auto pmr = std::make_shared<PassiveMetricsRegistry>();
prometheus::Exposer exposer(// endpoint);
exposer.RegisterCollectable(// normal registry);
exposer.RegisterCollectable(pmr);

But it's worth that opentelemetry-cpp provides this feature officially.

[timwoj]

We're not using otel-cpp for various reasons, instead preferring to use prometheus-cpp directly. I implemented this in a fork as zeek@2fec720, but never got around to opening a PR for it.

I agree that it's good for otel-cpp to provide this functionality directly themselves as well, since it has more uses there than just with Prometheus.