Increase the visibility of the api/key package (#650)

* Point to the convenience functions in api/key package

This is to increase the visibility of the api/key package through the
api/core package, otherwise developers often tend to miss the api/key
package altogether and write `core.Key(name).TYPE(value)` and complain
at the verbosity of such a construction. The api/key package would
allow them to write `key.TYPE(name, value)`.

* Use the api/key package where applicable

This transforms all the uses of `core.Key(name).TYPE(value)` to
`key.TYPE(name, value)`. This also should help increasing the
visibility of the api/key package for developers reading the otel-go
code.

Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>

example/basic/main.go

@@ -47,7 +47,7 @@ func initTracer() {
 	}
 	tp, err := sdktrace.NewProvider(sdktrace.WithSyncer(exp),
 		sdktrace.WithConfig(sdktrace.Config{DefaultSampler: sdktrace.AlwaysSample()}),
-		sdktrace.WithResourceAttributes(core.Key("rk1").String("rv11"), core.Key("rk2").Int64(5)))
+		sdktrace.WithResourceAttributes(key.String("rk1", "rv11"), key.Int64("rk2", 5)))
 	if err != nil {
 		log.Panicf("failed to initialize trace provider %v", err)
 	}