parse Prometheus metrics from JavaScript

prometheus-exposition.js

class Metric {
    /**
     * @param {string} help
     * @param {string} value
     * @param {string[]} values
     * @param {string[]} labels
     */
    constructor(help, value, values, labels) {
        this.help = help
        this.value = value
        this.values = values
        this.labels = labels
    }
}

/**
 * @param {string} metricsText
 * @returns {Map<string, Metric>}
 */
function parsePrometheusMetrics(metricsText) {
    let metricsLines = metricsText.split("\n")
    let metrics = new Map()
    for (let metricsLinesKey in metricsLines) {
        let metricsLine = metricsLines[metricsLinesKey]
        if (metricsLine === "") {
            continue
        }
        if (metricsLine.startsWith("# HELP ")) {
            let metricNameEndPos = metricsLine.substr(7).indexOf(" ")
            let metricsName = metricsLine.substr(7, metricNameEndPos)
            let metricsHelp = metricsLine.substr(7 + metricNameEndPos + 1)
            metrics[metricsName] = new Metric(metricsHelp, "", null, null)
        } else if (metricsLine.startsWith("# TYPE ")) {
            // not interesting
        } else {
            // name with labels
            let metricNameFqEndPos = metricsLine.lastIndexOf(" ")
            let metricsValue = metricsLine.substr(metricNameFqEndPos + 1)
            // "go_threads" or "agw_req{SSP="ssp1",result="accepted"}"
            let metricsNameFq = metricsLine.substr(0, metricNameFqEndPos)
            let metricsName
            let labelsStart = metricsNameFq.indexOf("{")
            if (labelsStart < 0) {
                metricsName = metricsNameFq
                let metric = metrics[metricsName];
                if (!metric) {
                    metric = new Metric(null, null, null, null)
                    metrics[metricsName] = metric
                }
                metric.value = metricsValue
            } else {
                metricsName = metricsNameFq.substr(0, labelsStart)
                let metricsLabels = metricsNameFq.substr(labelsStart + 1, metricsNameFq.length - 2)
                let metric = metrics[metricsName];
                if (!metric) {
                    metric = new Metric(null, null, null, null)
                    metrics[metricsName] = metric
                }
                metric.labels ||= []
                metric.labels.push(metricsLabels)
                metric.values ||= []
                metric.values.push(metricsValue)
            }
        }
    }
    return metrics
}

/**
 * @param metrics {Map<string, Metric>}
 * @param metricName {string}
 * @param labelKey {string}
 * @param labelVal {string}
 * @return string
 */
function getMetricVal(metrics, metricName, labelKey, labelVal) {
    let metric = metrics[metricName]
    if (!metric || !metric.labels) return null
    let needle = `"${labelKey}"="${labelVal}"`
    for (let i = 0; i < metric.labels.length; i++) {
        let labelStr = metric.labels[i]
        if (labelStr.includes(needle)) {
            return metric.values[i]
        }
    }
    return null
}

/**
 * @param metric {Metric}
 * @param labelNeedles {string}
 * @return {string}
 */
function getMetricValByLabel(metric, ...labelNeedles) {
    for (let i = 0; i < metric.labels.length; i++) {
        let labelStr = metric.labels[i]
        if (labelsMatched(labelNeedles, labelStr)) {
            return metric.values[i]
        }
    }
    return null
}

/**
 * @param metric {Metric}
 * @param labelNeedles {string}
 * @return {string[]}
 */
function filterMetricValByLabel(metric, ...labelNeedles) {
    let matchedValues = []
    for (let i = 0; i < metric.labels.length; i++) {
        let labelStr = metric.labels[i]
        if (labelsMatched(labelNeedles, labelStr)) {
            matchedValues.push(metric.values[i])
        }
    }
    return matchedValues
}

function labelsMatched(labelNeedles, labelStr) {
    let allMatched = true
    for (let j = 0; j < labelNeedles.length; j++) {
        let labelNeedle = labelNeedles[j]
        if (!labelStr.includes(labelNeedle)) {
            allMatched = false
            break
        }
    }
    return allMatched
}