Create alertmanager.js (#808)

* Create alertmanager.js

* Add changelog

* Rename contrib/alertmanager.js to contrib/jsTransformationFunctions/alertmanager.js

* Add docs to alertmanager.js

* Update alertmanager.js

* Update alertmanager.js with paragraphs

* Update alertmanager.js

convert `\n` line breaks to html `<br/>` so the upstream formatting is applied

---------

Co-authored-by: Will Hunt <will@half-shot.uk>

changelog.d/808.feature

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+Add generic webhook transformation JS snippet for Prometheus Alertmanager.

contrib/jsTransformationFunctions/alertmanager.js

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+/**
+ * This is a transformation function for Prometheus Alertmanager webhooks.
+ * https://prometheus.io/docs/alerting/latest/configuration/#webhook_config
+ *
+ * Creates a formatted `m.text` message with plaintext fallback, containing:
+ * - alert status and severity
+ * - alert name and description
+ * - URL to the entity that caused the alert
+ * The formatted message also contains a clickable link that silences the alert.
+ */
+
+/**
+ * @param status resolved or firing
+ * @param severity from the labels of the alert
+ * @returns colored text rendering of the status and severity
+ */
+function statusBadge(status, severity) {
+    let statusColor;
+    if (status === "resolved") {
+        return `<font color='green'><b>[RESOLVED]</b></font>`;
+    }
+
+    switch(severity) {
+        case 'resolved':
+        case 'critical':
+            return `<font color='red'><b>[FIRING - CRITICAL]</b></font>`;
+        case 'warning':
+            return `<font color='orange'><b>[FIRING - WARNING]</b></font>`;
+        default:
+            return `<b>[${status.toUpperCase()}]</b>`;
+    }
+}
+
+/**
+ * @param alert object from the webhook payload
+ * @param externalURL from the webhook payload
+ * @returns a formatted link that will silence the alert when clicked
+ */
+function silenceLink(alert, externalURL) {
+    filters = []
+    for (const [label, val] of Object.entries(alert.labels)) {
+        filters.push(encodeURIComponent(`${label}="${val}"`));
+    }
+    return `<a href="${externalURL}#silences/new?filter={${filters.join(",")}}">silence</a>`;
+}
+
+if (!data.alerts) { 
+    result = {
+        version: 'v2',
+        empty: true,
+    };
+    return;
+}
+
+const plainErrors = [];
+const htmlErrors = [];
+const { externalURL, alerts } = data;
+
+for (const alert of data.alerts) {
+    plainErrors.push(`**[${alert.status.toUpperCase()} - ${alert.labels.severity}]** - ${alert.labels.alertname}: ${alert.annotations.description} [source](${alert.generatorURL})`);
+    htmlErrors.push(`<p>${statusBadge(alert.status, alert.labels.severity)}</p><p><b>${alert.labels.alertname}</b>: ${alert.annotations.description.replaceAll("\n","<br\>")}</p><p><a href="${alert.generatorURL}">source</a> | ${silenceLink(alert, externalURL)}</p>`)
+    result = {
+        version: 'v2',
+        plain: plainErrors.join(`\n\n`),
+        html: htmlErrors.join(`<br/><br/>`),
+        msgtype: 'm.text'
+    };
+}