feat: add confidence intervals for SLO analysis visualizations

Part of #153 and https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/4774

Adds confidence intervals for SLO analysis dashboards.

Helpful in analysing SLOs, taking the confidence score into account.

What it looks like for a low RPS SLI (websockets/rails_request)

https://dashboards.gitlab.net/dashboard/snapshot/EWggMSWzKxcI18HkGENraZreOX97I5kD?var-environment=gprd&orgId=1&var-type=websockets&var-PROMETHEUS_DS=e8b842b2-d53b-460b-9125-f4a50db5fdcc&var-stage=main&var-component=rails_request&from=1716342274092&to=1716361726723

What is looks like for a high RPS service (web/rails_request)

There is only a very small margin between the confidence interval and the raw ratio

https://dashboards.gitlab.net/dashboard/snapshot/EWggMSWzKxcI18HkGENraZreOX97I5kD?var-environment=gprd&orgId=1&var-type=web&var-PROMETHEUS_DS=e8b842b2-d53b-460b-9125-f4a50db5fdcc&var-stage=main&var-component=rails_request&from=1716344275230&to=1716375779853