Add metrics to the Self monitoring default dashboard

Problem to solve

We would like to present a small number of metrics in the GitLab Self-Monitoring project and make the project expandable with more metrics as a self-service.

Further details

We should add 2 metrics for this dashboard and we should try to identify any gaps (if exist) in order to make this a self-service process.

Proposal

Based on #38038 (comment 260119683) we should add 2 charts to the default dashboard

1. Web Traffic - Error Ratio
2. API Traffic - Error Ratio

Documentation

We will need documentation on how to add dashboards and metrics. This is covered in #198048 (closed)

Success criteria

1. Default dashboard on the Self-monitoring project with 2 metrics
2. List of issues (if exist) for making this process self-service (also mentioned in #198048 (closed))

Links / references

Issue to add documentation: #198048 (closed)