fix(webvitals): Drop negative web vitals from measurements

[edwardgou-sentry]

Add allow_negative to BuiltinMeasurementKey. Filter out negative BuiltinMeasurements if allow_negative is false.

[jan-auer]

Thank you for this change, it makes sense to consider negative web vitals as invalid and remove them during normalization.

Instead of hard-coding this, we can use builtin_measurements from the global config. This way, the same normalization automatically applies to new measurements added in the future and extends beyond web vitals. Most of our measurements would require positive values (except for things measuring relative change, such as memory), so we can make this a new default:

/// Defines a builtin measurement.
#[derive(Debug, Default, Clone, Serialize, Deserialize, PartialEq, Hash, Eq)]
#[serde(default, rename_all = "camelCase")]
pub struct BuiltinMeasurementKey {
    name: String,
    unit: MetricUnit,
    #[serde(default, skip_serializing_if = "is_false")]
    allow_negative: bool,
}

fn is_false(b: &bool) -> bool {
    !b
}

There's already a function called remove_invalid_measurements that has access to this configuration and is used to prune invalid measurements. It is also called from within normalize_measurments (here), so the only thing left to do is move it to the bottom of the function after calling compute_measurements.

By returning false from the retain closure, you can evict measurements from the list. An example where builtin measurement keys are checked is here. You can add this additional check and return false if the measurement's value is negative and the matching config does not permit negative values.

[edwardgou-sentry]

Thanks @jan-auer ! I've made updates to BuiltinMeasurementKey an moved the negative check to remove_invalid_measurements

[jan-auer]

Thank you!