Limit max metrics embeds to 100

What does this MR do?

Redacts any metrics embeds in a markdown field past the first 100.

Context on the embedded metrics feature: https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#embedding-metric-charts-within-gitlab-flavored-markdown

Related issue: #14939 (closed)

Note that this change doesn't necessarily improve performance if embeds are distributed across multiple fields on a page.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [n/a] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team