Looks like I answered my question I believe hasn't been authorized yet, or
maybe I forgot to click 'post message'. But the solution is to pass the
label values as empty strings or with a hyphen placeholder like Brian
Candler suggests. Prometheus will pick these up and not show the label
name/value pair where an empty string or hyphen is for the value. Passing
in null values or creating separate metrics of the same name throw
exceptions.
Here is a snippet of my code for the nullable label names:
string[]? labels = new string[] { statusCode, method,
actionName ?? "", controllerName ?? "", path ?? "" };
I then pass the labels to a function that increments the counter by the
*length* parameter:
public void RegisterResponse(double length, params string[]
labelValues)
{
_responseCounter.WithLabels(labelValues).Inc( );
_responseLength.WithLabels(labelValues).Inc(length);
}
On Tuesday, August 16, 2022 at 3:05:52 AM UTC-4 Brian Candler wrote:
> > How do you deal with dynamic label names without having to create
> separate metrics counters?
>
> You have to have separate counters.
>
> Every combination of (metric name + set of label names/values) creates a
> distinct timeries, i.e. a different metric, by definition. Therefore you
> need different counters for each distinct combination of label values.
>
> Labels are not just somewhere to store a string. If you put
> high-cardinality values in a label, i.e. values which change from request
> to request and are not bounded to a small subset of values, then you will
> risk a "cardinality explosion" where you have millions of timeseries all
> with a single value. Under these conditions, Prometheus will crash and
> burn.
>
> In your case, if you're using controller name and action name for labels,
> then I think it should be OK as those are values chosen from a small set
> defined in code. For counting requests which don't have these fields, then
> use a dummy value like "-" for those labels.
>
> On Monday, 15 August 2022 at 22:34:16 UTC+1 [email protected] wrote:
>
>> In the default metrics exported with app.usehttpmetrics(), certain
>> metrics, such as http_requests_received_total will have scraped data that
>> only contain ''code' and 'method' labels which I see is defined here in
>> HttpRequestLabelNames.cs
>> <https://github.com/prometheus-net/prometheus-net/blob/38d45fa0a102e9afc76f031675aa0719d4f063d1/Prometheus.AspNetCore/HttpMetrics/HttpRequestLabelNames.cs>.
>>
>> I'm trying to replicate the same thing with custom middleware that records
>> the response body content length. But when requests with bad query params
>> return 404 responses there is no controller, endpoint, or action fields in
>> HttpContext and I can't use my custom counters with those label names
>> specified. How do you deal with dynamic label names without having to
>> create separate metrics counters?
>
>
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