Enable support for instrumentation (Page Inspector) for TagHelpers

[NTaylorMullen]

In TagHelpers initial state they will not support page inspector (instrumentation). They will however work around the pre-existing instrumentation. Meaning pieces of a TagHelpers body (if not ContentBehavior.Modify) will still communicate with page inspector via instrumentation.

We need to add support for page inspector in TagHelpers. However; it's not as simple as just adding it, we need to decide how much page inspector integration we'll have with TagHelpers.

[danroth27]

From @jodavis

Functional issue:
When content is generated by a tag helper (or even left unmodified by a tag helper), Browser Link can't map that element back to source.

Repro steps:

1. In a DNX website using MVC, add an IPageExecutionListenerFeature
2. Set a breakpoint in PageExecutionContext.BeginContext and EndContext
3. Run a Razor page that uses Tag Helpers.

Expected result:
BeginContext and EndContext are called for every range of the rendered page

Actual result:
BeginContext and EndContext are called for most of the rendered page, but not the parts rendered by Tag Helpers.