Move test metrics export configuration helper to gitlab-rspec gem

What does this MR do and why?

This change moves the custom metrics export helper class in to gitlab-rspec gem which allows to export metrics automatically in all rspec executions as long as needed variables are present.

• Use more generic variable names given clickhouse instance is not exclusively used for test metrics
• Move the configuration helper class to gitlab-rspec gem which allows better reuse and adding exports of test results for other gems

References

Closes Rename ClickHouse environment variables to use ... (gitlab-org/quality/analytics/team#316 - closed), Move exporter configuration helper to gitlab-rs... (gitlab-org/quality/analytics/team#333 - closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Validation

• unit test export: https://gitlab.com/gitlab-org/gitlab/-/jobs/12367843507#L5182
• e2e test export: https://gitlab.com/gitlab-org/gitlab/-/jobs/12367927700#L1197
• gem test export: https://gitlab.com/gitlab-org/gitlab/-/jobs/12369815665#L1169