Does this issue relate to a new feature or an existing bug?

• Bug
• New Feature

What version of Serilog is affected? Please list the related NuGet package.
2.8.0 (currently the latest)

What is the target framework and operating system? See target frameworks & net standard matrix.

All or NA

Please describe the current behavior?
The only constructor of MessageTemplateTextFormatter requires as its second and last argument a reference to an instance of IFormatProvider. However, the documentation says that this reference is allowed to be null. Indeed, MessageTemplateTextFormatter eventually passes this reference into four functions and all them syntactically specify that a reference to an instance of IFormatProvider is optional by providing the default value null.

Please describe the expected behavior?
I was expecting that the constructor of MessageTemplateTextFormatter would also syntactically specify that a reference to an instance of IFormatProvider is optional by providing the default value null.

If it would be helpful, I can create a PR to resolve this issue.