Fix outdated attribute examples in comments

[jack-berg]

opentelemetry-proto/opentelemetry/proto/trace/v1/trace.proto

Lines 199 to 211 in 84b3017

	// attributes is a collection of key/value pairs. Note, global attributes
	// like server name can be set using the resource API. Examples of attributes:
	//
	// "/http/user_agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"
	// "/http/server_latency": 300
	// "example.com/myattribute": true
	// "example.com/score": 10.239
	//
	// The OpenTelemetry API specification further restricts the allowed value types:
	// https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/README.md#attribute
	// Attribute keys MUST be unique (it is not allowed to have more than one
	// attribute with the same key).
	repeated opentelemetry.proto.common.v1.KeyValue attributes = 9;

Also discussed: #534 (comment)