mixins: Add code/grpc-code dimension to error widgets

[douglascamata]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• Changed HTTP/gRPC error widgets to use a new function that can slice them by code/grpc-code (respectively). This allows the label of the series in the error charts to be much more useful than they are today.

Screenshots

Store gRPC error widget before changes

Store gRPC error widget after changes

Verification

Used the output query in my team's live Grafana dashboard.

[fpetkovski]

👍

[douglascamata]

Leaving a decision documented: I created this qpsErrTotalPerLabelPanel function because in many places qpsErrTotalPanel is used for generic error charts (i.e. in the Compactor and Ruler dashboards).

As future work we could investigate whether those widgets have some sort of code/reason/message field that is worth surfacing in the widget's legend.

[douglascamata]

OMG I messed up a merge conflict solution here. Fixing it.

[douglascamata]

Looks like we have a flake unit test?

--- FAIL: TestMultiTSDBPrune (6.14s)
    --- FAIL: TestMultiTSDBPrune/prune_tsdbs_with_object_storage (3.07s)
        testutil.go:91: multitsdb_test.go:475: ""
            
            	exp: 2
            
            	got: 1
            
FAIL

[douglascamata]

Build's all green, this is good to go finally 🙇

[fpetkovski]

Thanks!