query: handle query.Analyze returning nil gracefully

[Saumya40-codes]

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

Changes

Should fix #8198

In engine query.Analyze returns nil if query.exec doesn't implements telemetry.ObservableVectorOperator. This PR handles it gracefully as later o/p of it excesses the respective fields

Verification

Not exactly how tagged issue might be stating but if try to analyze the query when its of type [noop] (with query.mode=distributed), the similar type of behaviour occurs (which also gets handled)

[MichaHoffmann]

In engine query.Analyze returns nil if query.exec doesn't implements telemetry.ObservableVectorOperator. This PR handles it gracefully as later o/p of it excesses the respective fields

can we fix this in the engine? it should not return nil probably

[MichaHoffmann]

It seems like a bug where not all nodes implement

type ObservableVectorOperator interface {
	model.VectorOperator
	OperatorTelemetry
}

[Saumya40-codes]

In engine query.Analyze returns nil if query.exec doesn't implements telemetry.ObservableVectorOperator. This PR handles it gracefully as later o/p of it excesses the respective fields

can we fix this in the engine? it should not return nil probably

currently it is

func (q *Query) Analyze() *AnalyzeOutputNode 

ig we change it to return maybe (*AnalyzeOutputNode , error) (err if a node isnt implementing ObservableVectorOperator in case)

[Saumya40-codes]

It seems like a bug where not all nodes implement

type ObservableVectorOperator interface {
	model.VectorOperator
	OperatorTelemetry
}

Yes, noop operator misses that interface field (rest of what i saw implements it)

type operator struct {
	model.VectorOperator
	// telemetry.OperatorTelemetry <- Needs to be added
}

func NewOperator(opts *query.Options) model.VectorOperator {
	scanner := prometheus.NewVectorSelector(
		model.NewVectorPool(0),
		noopSelector{},
		opts,
		0, 0, false, 0, 1,
	)
	return &operator{VectorOperator: scanner}
}

maybe based on the original issue the below might've got triggered

if len(remoteQueries) == 0 {
	return Noop{}
}