Unable to prefix names of default metrics in v9 as `prefix` option from v8.0.0 is not a thing in v9

[skellla]

Unable to prefix names of default metrics in v9 as `prefix` option from v8.0.0 is not a thing in v9

[bhovhannes]

After looking into sources of both fastify-metrics and prom-client I figured out how to configure what I want. Posting it here in case someone will face the same problems as me during upgrade.

So, if with v8 we wrote:

app.register(require(fastify-metrics), {
    prefix: 'app_',
    blacklist: [
        `/internal/metrics`,
        `/internal/health/readiness`
    ],
    enableDefaultMetrics: true,
    enableRouteMetrics: true
})

in v9 this should be written as:

app.register(require("fastify-metrics"), {
    defaultMetrics: {
        enabled: true, // this is the default value and can be omitted
        prefix: 'app_'
    },
    routeMetrics: {
        enabled: true, // this is the default value and can be omitted
        routeBlacklist: [
            `/internal/metrics`,
            `/internal/health/readiness`
        ],
        overrides: {
            histogram: {
                name: `app_http_request_duration_seconds`
            },
            summary: {
                name: `app_http_request_summary_seconds`
            }
        },
    }
})

• Essentially, all options under defaultMetrics key are being passed to prom-client, including prefix.
• To add prefix to request histogram and summary, one should use overrides key and provide a prefixed name. v9 does not use prefix passed for default metrics for request histogram and summary.

[SkeLLLa]

Configuration object in v9 changed and there's no compatibility between v8 and v9. See https://github.com/SkeLLLa/fastify-metrics/blob/master/docs/fastify-metrics.idefaultmetricsconfig.md, it extends DefaultMetricsCollectorConfiguration from prom-client where prefix is configured.