Fix exported instrument kind const value change (#5385)

#5304 introduced the following incompatible changes:

- `InstrumentKindObservableCounter`: value changed from 4 to 5
- `InstrumentKindObservableGauge`: value changed from 6 to 7
- `InstrumentKindObservableUpDownCounter`: value changed from 5 to 6

This reverts that change, making `InstrumentKindGauge` explicitly `7`.

Additionally, this removes the use of `iota` to prevent this kind of
breaking change from being accidentally introduced in the future.

sdk/metric/instrument.go

@@ -30,32 +30,32 @@ type InstrumentKind uint8
 const (
 	// instrumentKindUndefined is an undefined instrument kind, it should not
 	// be used by any initialized type.
-	instrumentKindUndefined InstrumentKind = iota // nolint:deadcode,varcheck,unused
+	instrumentKindUndefined InstrumentKind = 0 // nolint:deadcode,varcheck,unused
 	// InstrumentKindCounter identifies a group of instruments that record
 	// increasing values synchronously with the code path they are measuring.
-	InstrumentKindCounter
+	InstrumentKindCounter InstrumentKind = 1
 	// InstrumentKindUpDownCounter identifies a group of instruments that
 	// record increasing and decreasing values synchronously with the code path
 	// they are measuring.
-	InstrumentKindUpDownCounter
+	InstrumentKindUpDownCounter InstrumentKind = 2
 	// InstrumentKindHistogram identifies a group of instruments that record a
 	// distribution of values synchronously with the code path they are
 	// measuring.
-	InstrumentKindHistogram
-	// InstrumentKindGauge identifies a group of instruments that record
-	// instantaneous values synchronously with the code path they are
-	// measuring.
-	InstrumentKindGauge
+	InstrumentKindHistogram InstrumentKind = 3
 	// InstrumentKindObservableCounter identifies a group of instruments that
 	// record increasing values in an asynchronous callback.
-	InstrumentKindObservableCounter
+	InstrumentKindObservableCounter InstrumentKind = 4
 	// InstrumentKindObservableUpDownCounter identifies a group of instruments
 	// that record increasing and decreasing values in an asynchronous
 	// callback.
-	InstrumentKindObservableUpDownCounter
+	InstrumentKindObservableUpDownCounter InstrumentKind = 5
 	// InstrumentKindObservableGauge identifies a group of instruments that
 	// record current values in an asynchronous callback.
-	InstrumentKindObservableGauge
+	InstrumentKindObservableGauge InstrumentKind = 6
+	// InstrumentKindGauge identifies a group of instruments that record
+	// instantaneous values synchronously with the code path they are
+	// measuring.
+	InstrumentKindGauge InstrumentKind = 7
 )
 
 type nonComparable [0]func() // nolint: unused  // This is indeed used.

sdk/metric/pipeline_registry_test.go

@@ -145,14 +145,14 @@ func testCreateAggregators[N int64 | float64](t *testing.T) {
 	)
 
 	instruments := []Instrument{
-		{Name: "foo", Kind: InstrumentKind(0)}, // Unknown kind
-		{Name: "foo", Kind: InstrumentKindCounter},
-		{Name: "foo", Kind: InstrumentKindUpDownCounter},
-		{Name: "foo", Kind: InstrumentKindHistogram},
-		{Name: "foo", Kind: InstrumentKindGauge},
-		{Name: "foo", Kind: InstrumentKindObservableCounter},
-		{Name: "foo", Kind: InstrumentKindObservableUpDownCounter},
-		{Name: "foo", Kind: InstrumentKindObservableGauge},
+		InstrumentKind(0):                     {Name: "foo", Kind: InstrumentKind(0)}, // Unknown kind
+		InstrumentKindCounter:                 {Name: "foo", Kind: InstrumentKindCounter},
+		InstrumentKindUpDownCounter:           {Name: "foo", Kind: InstrumentKindUpDownCounter},
+		InstrumentKindHistogram:               {Name: "foo", Kind: InstrumentKindHistogram},
+		InstrumentKindGauge:                   {Name: "foo", Kind: InstrumentKindGauge},
+		InstrumentKindObservableCounter:       {Name: "foo", Kind: InstrumentKindObservableCounter},
+		InstrumentKindObservableUpDownCounter: {Name: "foo", Kind: InstrumentKindObservableUpDownCounter},
+		InstrumentKindObservableGauge:         {Name: "foo", Kind: InstrumentKindObservableGauge},
 	}
 
 	testcases := []struct {