Refactor WriteAttribute API surface.

[pranavkm]

Today we generate a single WriteAttribute method which contains values that are only used for instrumentation. The suggestion here is to instead have two methods

• WriteAttribute which does not carry any instrumentation information. I'll update this method's signature once I investigate.
• WriteInstrumentedAttribute which carries instrumentation information. This method will look identical to the current WriteAttribute.

The primary motive for this method is to have minimal runtime effects when instrumentation (BrowserLink) is not enabled.

[yishaigalatzer]

@pranavkm if we already addressed this issue, please close. Otherwise move to RC

[pranavkm]

We decided an alternative would be to replace the structures with multiple individual calls

For instance:

Before

WriteAttribute("class", 
  Tuple.Create(" class=\"", 74), 
  Tuple.Create("\"", 86), 
  Tuple.Create(Tuple.Create("", 82), 
  Tuple.Create<System.Object, System.Int32>(cls, 82), 
  false));

After

BeginWriteAttribute("class", " class=\"", 74, "\"", 86, 1);
WriteAttributeValue("", 82, cls, 82, 4, false);
EndWriteAttribute();

Since attributes cannot be interleaved, tracking state in Mvc is simple. This avoids having different code paths for instrumented and non-instrumented hosts without the overhead of allocating structures.