scope: add RawVal type for raw float64 values

Adds a new RawVal type to store raw float64 values without any aggregation (histogram, sum, etc).
This provides a simpler alternative to FloatVal when only the most recent value is needed.

With high cardinality data, it's important to use only minimal fields (especially as Prometheus/Victoria can calculate the sum/min/max aggregations)

Author: elek

examples/minimal/main.go

@@ -50,6 +50,8 @@ func ComputeThing(ctx context.Context, arg1, arg2 int) (res int, err error) {
 		mon.Event("hit 3")
 	}
 
+	mon.RawVal("raw").Observe(1.0)
+	mon.RawValk(monkit.NewSeriesKey("rawk").WithTag("foo", "bar"), monkit.Sum, monkit.Count).Observe(1.0)
 	mon.BoolVal("was-4").Observe(res == 4)
 	mon.IntVal("res").Observe(int64(res))
 	mon.DurationVal("took").Observe(time.Second + time.Duration(rand.Intn(int(10*time.Second))))

scope.go

@@ -233,6 +233,29 @@ func (s *Scope) DurationVal(name string, tags ...SeriesTag) *DurationVal {
 	return m
 }
 
+// RawValk retrieves or creates a RawVal with a given key and aggregations.
+func (s *Scope) RawValk(key SeriesKey, aggregations ...Aggregate) *RawVal {
+	source := s.newSource(key.String(), func() StatSource {
+		return NewRawVal(key, aggregations...)
+	})
+	m, ok := source.(*RawVal)
+	if !ok {
+		panic(fmt.Sprintf("%s already used for another stats source: %#v", key, source))
+	}
+	return m
+}
+
+// RawVal retrieves or creates a RawVal after the given name and tags.
+func (s *Scope) RawVal(name string, tags ...SeriesTag) *RawVal {
+	return s.RawValk(NewSeriesKey(name).WithTags(tags...))
+}
+
+// RawValf retrieves or creates a RawVal after the given printf-formatted
+// name.
+func (s *Scope) RawValf(template string, args ...interface{}) *RawVal {
+	return s.RawVal(fmt.Sprintf(template, args...))
+}
+
 // Timer retrieves or creates a Timer after the given name.
 func (s *Scope) Timer(name string, tags ...SeriesTag) *Timer {
 	source := s.newSource(sourceName("", name, tags), func() StatSource {

val.go

@@ -246,3 +246,84 @@ func (v *DurationVal) Quantile(quantile float64) (rv time.Duration) {
 	v.mtx.Unlock()
 	return rv
 }
+
+// Aggregate can implement additional aggregation for collected values.
+type Aggregate func() (observe func(val float64), stat func() (field string, val float64))
+
+// RawVal is a simple wrapper around a float64 value without any aggregation
+// (histogram, sum, etc). Constructed using NewRawVal, though its expected usage is like:
+//
+//	var mon = monkit.Package()
+//
+//	func MyFunc() {
+//	  ...
+//	  mon.RawVal("value").Observe(val)
+//	  ...
+//	}
+type RawVal struct {
+	mtx       sync.Mutex
+	value     float64
+	key       SeriesKey
+	stats     []func() (field string, val float64)
+	observers []func(val float64)
+}
+
+// NewRawVal creates a RawVal
+func NewRawVal(key SeriesKey, aggregations ...Aggregate) *RawVal {
+	val := &RawVal{key: key}
+	for _, agg := range aggregations {
+		observe, stat := agg()
+		val.stats = append(val.stats, stat)
+		val.observers = append(val.observers, observe)
+	}
+	return val
+}
+
+// Observe sets the current value
+func (v *RawVal) Observe(val float64) {
+	v.mtx.Lock()
+	v.value = val
+	for _, o := range v.observers {
+		o(val)
+	}
+	v.mtx.Unlock()
+}
+
+// Stats implements the StatSource interface.
+func (v *RawVal) Stats(cb func(key SeriesKey, field string, val float64)) {
+	v.mtx.Lock()
+	cb(v.key, "recent", v.value)
+	v.mtx.Unlock()
+	for _, s := range v.stats {
+		field, value := s()
+		cb(v.key, field, value)
+	}
+}
+
+// Get returns the current value
+func (v *RawVal) Get() float64 {
+	v.mtx.Lock()
+	value := v.value
+	v.mtx.Unlock()
+	return value
+}
+
+// Count is a value aggregator that counts the number of times the value is measured.
+func Count() (observe func(val float64), stat func() (field string, val float64)) {
+	var counter int
+	return func(val float64) {
+			counter++
+		}, func() (field string, val float64) {
+			return "count", float64(counter)
+		}
+}
+
+// Sum is a value aggregator that summarizes the values measured.
+func Sum() (observe func(val float64), stat func() (field string, val float64)) {
+	var sum int
+	return func(val float64) {
+			sum += int(val)
+		}, func() (field string, val float64) {
+			return "sum", float64(sum)
+		}
+}