This script can act as a middleman between Sentry's webhook system and Google Chat Incoming Webhooks

sentry_google_chat_webhook.php

<?php

$googleChatWebhookURL = '<YOUR-GOOGLE-CHAT-WEBHOOK-URL>';

$integrationClientSecret = '<YOUR-Sentry-Settings-integration-Edit Integration-CREDENTIALS-Client Secret>';

/**
 * https://developers.google.com/chat/how-tos/webhooks#step_2_write_the_webhook_script
 * https://developers.google.com/chat/api/guides/message-formats/basic
 */
function getSimpleChatMessage(array $params): array
{
    return [
        'text' => sprintf('🚨 <%s|Sentry issue>: `%s` 📣 *%s count* 🚨' . PHP_EOL . '```%s```',
            getSentryIssueURL($params),
            $params['issueId'],
            number_format($params['count'], 0, null, ' '),
            $params['metadataValue'],
        )
    ];
}

/**
 * https://developers.google.com/chat/api/guides/message-formats/cards
 * TODO: SOON
 */
function getCardChatMessage(array $params): array
{
    return [];
}

function verifySignature(string $body, string $signature, string $secret = ''): bool
{
    return (getHash($secret, $body) === $signature);
}

function getHash(string $secret, string $content): string
{
    return hash_hmac('sha256', $content, utf8_encode($secret));
}

/**
 * @throws JsonException
 */
function getSentryData(string $content): array
{
    return json_decode($content, true, 512, JSON_THROW_ON_ERROR);
}

/**
 * https://docs.sentry.io/product/integrations/integration-platform/webhooks/issues/
 */
function getChatMessageParams(array $json): array
{
    $issue = $json['data']['issue'];

    return [
        'organisation' => $issue['project']['name'],
        'projectId' => $issue['project']['id'],

        'issueId' => $issue['id'],
        'title' => $issue['title'],
        'metadataValue' => $issue['metadata']['value'],
        'count' => $issue['count'],
    ];
}

function getSentryIssueURL(array $params): string
{
    return sprintf('https://sentry.io/organizations/%s/issues/%s/?project=%s',
        $params['organisation'],
        $params['issueId'],
        $params['projectId'],
    );
}

/**
 * @throws JsonException
 */
function sendGoogleChatMessage(string $webhookURL, array $message): void
{
    $ch = curl_init($webhookURL);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($message, JSON_THROW_ON_ERROR));
    curl_setopt( $ch, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
    curl_exec($ch);
}

$sentrySignature = $_SERVER['HTTP_SENTRY_HOOK_SIGNATURE'] ?? '';
$sentryBodyContent = (string)file_get_contents('php://input');
if ( ! verifySignature($sentryBodyContent, $sentrySignature, $integrationClientSecret)) {
    header('Location: /');
    exit;
}

$jsonData = getSentryData($sentryBodyContent);
$params = getChatMessageParams($jsonData);
$message = getSimpleChatMessage($params);
sendGoogleChatMessage($googleChatWebhookURL, $message);

Preview example: