meter: add option helper and provide default metric name and label prefix

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

meter/context.go

@@ -0,0 +1,34 @@
+package meter
+
+import (
+	"context"
+)
+
+type meterKey struct{}
+
+// FromContext get meter from context
+func FromContext(ctx context.Context) (Meter, bool) {
+	if ctx == nil {
+		return nil, false
+	}
+	c, ok := ctx.Value(meterKey{}).(Meter)
+	return c, ok
+}
+
+// NewContext put meter in context
+func NewContext(ctx context.Context, c Meter) context.Context {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	return context.WithValue(ctx, meterKey{}, c)
+}
+
+// SetOption returns a function to setup a context with given value
+func SetOption(k, v interface{}) Option {
+	return func(o *Options) {
+		if o.Context == nil {
+			o.Context = context.Background()
+		}
+		o.Context = context.WithValue(o.Context, k, v)
+	}
+}