Add shared credentials for Logmatic (logmatic.io → datadoghq.com)

[mangeshraut712]

Bar check: This meets the shared-credentials bar. Logmatic.io and Datadog are the same product. logmatic.io/login redirects to app.datadoghq.com/account/login, which returns 200 with the title Datadog: Log In, and a nonexistent path on that host returns 404. A password saved on logmatic.io is the password for that same account on datadoghq.com.

The old apex redirects to the Datadog site, so logmatic.io is marked obsolete.

Evidence, checked live:

• https://logmatic.io/login → 301 https://www.datadoghq.com/login → 301 https://app.datadoghq.com/ → 307 https://app.datadoghq.com/account/login → 200, title Datadog: Log In. The initial HTML has no type="password" input; the page is the Datadog login app and names a standard login method plus /account/forgot_password.
• https://app.datadoghq.com/account/login → 200 with the same title.
• https://app.datadoghq.com/this-path-should-not-exist-4a36 → 404
• https://logmatic.io/ → 301 https://www.datadoghq.com/ (title Cloud Monitoring as a Service | Datadog)

Overall Checklist

• I agree to the project's Developer Certificate of Origin
• The top-level JSON objects are sorted alphabetically
• There are no open pull requests for the same update

for shared-credentials.json

• There's evidence the domains are currently related (logmatic.io/login redirects to the Datadog login)
• If using from and to, the from domain redirects to the to domain to log in

[mangeshraut712]

Closing this. logmatic.io/login redirects to the Datadog login. I have not verified that a password saved on Logmatic is accepted by Datadog. A redirect to the acquirer's login is not enough.