Baggage calls validate() on Member too often

[puckpuck]

Description

validate() is called on a Member when it is created, and when the Member is added to Baggage.

Environment

• OS: linux
• Architecture: amd64
• Go Version: 1.17
• opentelemetry-go version: 1.3.0

Steps To Reproduce

b := baggage.FromContext(ctx)
m, _ := baggage.NewMember("foo", "bar") // validate() is called on Member
b.SetMember(m) // validate() is called on Member

Expected behavior

validate() is only called at Member creation or when Member is added to a Baggage. Since Tracing is adding overhead to an application, this is to minimize all overhead as much as possible.