Improve performance of metric expression matcher (#5864)

Refactored env struct to avoid creating closure functions for every
expression evaluation operation. This reduced the allocations and
increased the speed.

Improvements:

$ benchstat old.txt new.txt
name          old time/op    new time/op    delta
ExprFilter-8     537µs ± 2%     476µs ± 1%  -11.33%  (p=0.000 n=9+10)

name          old alloc/op   new alloc/op   delta
ExprFilter-8     151kB ± 0%     102kB ± 0%  -32.43%  (p=0.000 n=8+8)

name          old allocs/op  new allocs/op  delta
ExprFilter-8     8.44k ± 0%     6.39k ± 0%  -24.24%  (p=0.000 n=10+10)

internal/coreinternal/processor/filterexpr/matcher.go

@@ -27,9 +27,17 @@ type Matcher struct {
 
 type env struct {
 	MetricName string
-	// TODO: replace this with GetLabel func(key string) (string,bool)
-	HasLabel func(key string) bool
-	Label    func(key string) string
+	attributes pdata.AttributeMap
+}
+
+func (e *env) HasLabel(key string) bool {
+	_, ok := e.attributes.Get(key)
+	return ok
+}
+
+func (e *env) Label(key string) string {
+	v, _ := e.attributes.Get(key)
+	return v.StringVal()
 }
 
 func NewMatcher(expression string) (*Matcher, error) {
@@ -100,21 +108,14 @@ func (m *Matcher) matchEnv(metricName string, attributes pdata.AttributeMap) (bo
 	return m.match(createEnv(metricName, attributes))
 }
 
-func createEnv(metricName string, attributes pdata.AttributeMap) env {
-	return env{
+func createEnv(metricName string, attributes pdata.AttributeMap) *env {
+	return &env{
 		MetricName: metricName,
-		HasLabel: func(key string) bool {
-			_, ok := attributes.Get(key)
-			return ok
-		},
-		Label: func(key string) string {
-			v, _ := attributes.Get(key)
-			return v.StringVal()
-		},
+		attributes: attributes,
 	}
 }
 
-func (m *Matcher) match(env env) (bool, error) {
+func (m *Matcher) match(env *env) (bool, error) {
 	result, err := m.v.Run(m.program, env)
 	if err != nil {
 		return false, err

internal/coreinternal/processor/filterexpr/matcher_test.go

@@ -30,7 +30,7 @@ func TestCompileExprError(t *testing.T) {
 func TestRunExprError(t *testing.T) {
 	matcher, err := NewMatcher("foo")
 	require.NoError(t, err)
-	matched, _ := matcher.match(env{})
+	matched, _ := matcher.match(&env{})
 	require.False(t, matched)
 }