logging: Tag Helpers: MVC Tag Helpers should log warnings

[DamianEdwards]

Generally speaking, Tag Helpers should only throw runtime exceptions due to bad input, e.g. the AnchorTagHelper should throw if the value specified for asp-controller isn't a valid controller name.

However there are many other cases where it would be helpful to notify the user of the helper's decision to no-op or otherwise, even when it isn't an error case, e.g. <label asp-for="FirstName" for="FirstName">First Name:</label> this will effectively no-op the LabelTagHelper as the for attribute and label display text have been specifically supplied and will be honored.

In these cases the Tag Helpers should log a warning. It should be done in such a way that someone viewing the logs could filter to show all warnings from all the MVC Tag Helpers, or specific Tag Helpers, e.g. only show warnings from the InputTagHelper.

[yishaigalatzer]

Need to be specific about the trace level we want here.

We got to be careful when selecting the level, as for example Warning might be too verbose, for the cases exposed above.

[kichalla]

Commit 0a1918a