Allow custom tags about express-prom-bundle HOT 6 CLOSED

Hi @ericuldall
Thanks for the contribution! I decided for a little simpler solution though, introducing two different metrics: customLabels and transformLabels. The docs were extended accordingly.

I removed validation, since I expect the developers to understand what they're doing ;) The validation is needed where the software's decisions are not straight forward.

Your example would look like this now:

const metricsMiddleware = promBundle({includeMethod: true, customLabels: customTags});

... and transformLabels can be used for a dynamicly changing labels. See https://github.com/jochen-schweizer/express-prom-bundle/blob/master/advanced-example.js

Hope it's not too much of a change compared to your intention.

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Is the update available on npm?

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I see the update to the docs there. Thanks for adding this! It will be very useful.

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Is it possible to use transformLabels to add a custom tag depending on the response body?

In my case I need a custom label to track GraphQL error metrics. And the list of errors is returned in the response body together with the data.

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why not? The signature is: transformLabels: function(labels, req, res). So if you accumulate the data you need in either of req or res, then you can also can transform the labels accordingly.

I'd say res.locals is a fine place for the metrics.

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My question was a bit premature, sorry.

It was born from the lack of understanding of how express works, and the intricacies of reading a response body in a middleware.

I did end up putting errors from response body on to res.locals and then transforming a custom label using using their values. Thank you.

If anyone stumbles upon this issue in the same situation I was in, I used this to read the body:

function(res) {
    var end = res.end;
    res.end = function(chunk) {
        res.locals.errors = undefined;
        if (chunk) {
            try {
                res.locals.errors = JSON.parse(chunk.toString('utf-8')).errors;
            } catch {
            }
        }
        end.apply(this, arguments);
    }
}

Inspired by this answer https://stackoverflow.com/a/33881887/6493014

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