chore(datadog): add metrics for malicious file or link activity

[halfwhole]

Problem

Currently, the metrics for malicious activity are defined under metric filters for CloudWatch log groups. (See AWS console for more details, requires AWS permissions to access)

The current approach presents several problems:

1. We cannot distinguish between malicious links and malicious files being uploaded
2. We cannot distinguish between staging/prod and gov/edu/health services without giving each metric a different name
3. The metrics are not defined in the codebase, so it's not under version control, and it adds some 'hidden' technical complexity on AWS infra that's not obvious to developers

Solution

Add two new malicious activity metrics that will be sent to Datadog: one for malicious files (go.malicious_activity.file), and one for malicious links (go.malicious_activity.link). The env and service tags for staging/prod and gov/edu/health respectively should be added on Datadog automatically. This will allow us to remove metric filters on CloudWatch in the future as well, if we want to.

Improvements:

Resolved a TODO comment from before (#1993 (comment)) in UrlManagementService.ts. For the shortlink creation metric, a new tag was also added to label the source (console or bulk, but not yet API).

Tests

• Test on staging to make sure that new metrics are being sent with tags

Deploy notes

Post-deployment: can remove CloudWatch metric filters for malicious activity (optional, but would be nice to do)

[gweiying]

lgtm!