Generic lambda should truncate long messages to fit slack limit

[eoinkelly]

Slack has a limit of 3000 chars on a section type. We should truncate long messages to fit because a truncated message is much better than no message and it will signal that the limit exists to the dev.

Rough example:

const buildSlackMsg = (event: unknown): IncomingWebhookSendArguments => {
  let warning = JSON.stringify(event, undefined, 2);

  // The maximum length of a `section.text` field in Slack API is 3000
  // characters (https://api.slack.com/reference/block-kit/blocks#section) Slack
  // Webhooks will HTTP 400 if the message we give it is too long.

  const TRUNCATED_TEXT_REPLACEMENT = '\n\n[MESSAGE TRUNCATED FOR SLACK]';
  const WRAPPER_TEXT_LENGTH = 12; // 6 quoted backticks
  const MAX_WARNING_LENGTH =
    3000 - WRAPPER_TEXT_LENGTH - TRUNCATED_TEXT_REPLACEMENT.length;

  if (warning.length > MAX_WARNING_LENGTH) {
    warning = warning.substring(0, MAX_WARNING_LENGTH);
    warning = warning.concat(TRUNCATED_TEXT_REPLACEMENT);
  }

  const alarmMessage = `\`\`\`${warning}\`\`\``;

  return {
    channel: process.env.SLACK_CHANNEL,
    blocks: [
      {
        type: 'header',
        text: {
          type: 'plain_text',
          text: ':warning: An event requires investigation',
          emoji: true
        }
      },
      {
        type: 'section',
        text: {
          type: 'mrkdwn',
          text: alarmMessage
        }
      }
    ]
  };
};