fix: skip exemplars for instant queries

[javiermolinar]

What this PR does:
Currently, we are computing the exemplars for query_range and instant queries. Since exemplars are not supported for instant queries (same as Prometheus) we can save that computing time.

It also fix a bug where the exemplars query param was not being honored

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[javiermolinar]

Can we live without this config?

tempo/modules/frontend/config.go

Line 96 in 280128f

Exemplars: false, // TODO: Remove?

If max_exemplars is set to 0 then it means that the Exemplars are disabled

[mdisibio]

Can we live without this config?

tempo/modules/frontend/config.go

Line 96 in 280128f

Exemplars: false, // TODO: Remove?

If max_exemplars is set to 0 then it means that the Exemplars are disabled

This sounds good to me. @mapno You originally implemented this, can you think of a scenario where max_exemplars=0 and exemplars=true is meaningful?

[mapno]

Can we live without this config?

tempo/modules/frontend/config.go

Line 96 in 280128f

Exemplars: false, // TODO: Remove?

If max_exemplars is set to 0 then it means that the Exemplars are disabled

This sounds good to me. @mapno You originally implemented this, can you think of a scenario where max_exemplars=0 and exemplars=true is meaningful?

Can't think of one. Possible was left from a previous implementation. I'm good to remove it.

[knylander-grafana]

Thank you for updating the docs. The updates look good.