fastfloat: support parsing "infinity" strings

This is needed for OpenMetrics - see https://github.com/OpenObservability/OpenMetrics/blob/master/OpenMetrics.md#abnf

Author: valyala

fastfloat/parse.go

@@ -237,7 +237,9 @@ func ParseBestEffort(s string) float64 {
 		if strings.HasPrefix(s, "+") {
 			s = s[1:]
 		}
-		if strings.EqualFold(s, "inf") {
+		// "infinity" is needed for OpenMetrics support.
+		// See https://github.com/OpenObservability/OpenMetrics/blob/master/OpenMetrics.md
+		if strings.EqualFold(s, "inf") || strings.EqualFold(s, "infinity") {
 			if minus {
 				return -inf
 			}

fastfloat/parse_test.go

@@ -308,6 +308,10 @@ func TestParseBestEffort(t *testing.T) {
 	f("-Inf", math.Inf(-1))
 	f("+iNf", math.Inf(1))
 	f("INF", math.Inf(1))
+	f("infinity", math.Inf(1))
+	f("-Infinity", math.Inf(-1))
+	f("+iNfINIty", math.Inf(1))
+	f("INFINITY", math.Inf(1))
 	f("nan", math.NaN())
 	f("-nan", math.NaN())
 	f("naN", math.NaN())