Include global metrics in the per-request collector registry

[candlerb]

This is a quick proof-of-concept fix for the zero metrics (snmp_packet_duration_seconds_bucket/sum/count, snmp_unexpected_pdu_type_total, snmp_packets_total, snmp_packet_retries_total) and missing metrics (snmp_collection_duration_seconds_count/sum, snmp_request_errors_total)

To consider:

• Do we really want the NxM metrics for snmp_collection_duration_seconds_count/sum (for every possible combination of auth and module, even those not being used?) Maybe this can be dropped given that Support fetching multiple modules in one scrape #945 will be introducing per-module scrape metrics?
• The code could be simplified a little by removing the metrics member from the Collector struct, and using globalMetrics directly wherever it's needed, instead of passing it around.

Fixes #950

[candlerb]

Aside: for some reason I get E-mail notifications of "golangci-lint workflow run" failing when I push to my own repo:

View workflow run

lint: generator/config.go#L1
: # github.com/prometheus/snmp_exporter/generator

lint: generator/net_snmp.go#L19
fatal error: net-snmp/net-snmp-config.h: No such file or directory

lint
issues found

However, the checks in the pull request pass.

[SuperQ]

Hmm, I'm not sure about this approach. I much prefer using promauto and passing a registry.

If we need to init some metrics, it's better to loop over things with metric.WithLabelValues(...).

As for the labels, I'm on the fence.

[candlerb]

Hmm, I'm not sure about this approach. I much prefer using promauto and passing a registry.

I think a global persistent registry makes a lot of sense, but I don't know how to do that given that the current architecture creates a disposable registry for every request, containing a dynamically-created collector:

func handler(w http.ResponseWriter, r *http.Request, logger log.Logger) {
        ...
        registry := prometheus.NewRegistry()
        c := collector.New(r.Context(), target, authName, auth, nmodules, logger, registry, *concurrency)
        registry.MustRegister(c)
        // Delegate http serving to Prometheus client library, which will call collector.Collect.
        h := promhttp.HandlerFor(registry, promhttp.HandlerOpts{})
        h.ServeHTTP(w, r)
}

Perhaps collector.Collect() could delegate to the global registry, is that what you mean? (Can a registry register another registry? I haven't checked this)

Aside: I wondered briefly if this is a use case for Unregister but ruled it out, since there could be concurrent scrapes with different auth and module parameters.

[SuperQ]

I figured it out, see #971.

[candlerb]

Being fixed in #971