Add total_relation =approx to query meta

[glookka]

Proposal:

When running a groupby query over an rt table with max_matches less than the number of groups, some groups may be omitted when disk chunk results are merged. This results in the total_found not reflecting the real number of unique groups while total_relation still shows "eq" which might be misleading.

Checklist:

^{To be completed by the assignee. Check off tasks that have been completed or are not applicable.}

• Implementation completed
• Tests developed
• Documentation updated
• Documentation reviewed
• OpenAPI YAML updated and issue created to rebuild clients

[Ansita20]

@glookka Can you please assign me this issue for hacktoberfest?

[sanikolaev]

@Ansita20 done

[sanikolaev]

@Ansita20 how's it going? Do you need any help?