Lazily import ILoggerProviders

[DustinCampbell]

Fixes #10326

This change makes our AbstractLoggerFactory take a set of Lazy<ILoggerProvider> instances and only realize them the first time a message is logged.

🤔 I wonder if it would be possible to change our ILoggerProvider instances to declare their minimum LogLevel via MEF and import that as metadata? That would allow us to avoid instantiating all the logger providers just to call IsEnabled.

[ryzngard]

🤔 I wonder if it would be possible to change our ILoggerProvider instances to declare their minimum LogLevel via MEF and import that as metadata? That would allow us to avoid instantiating all the logger providers just to call IsEnabled.

How would that work with runtime logging level change?

[DustinCampbell]

How would that work with runtime logging level change?

Yeah, maybe it couldn't be done. At least, I can't visualize how we'd make output window logger track the log level. I suppose we could make deeper changes to make the runtime log part of the core logger infrastructure.

[davidwengier]

This is awesome. In the meeting I was thinking "We'll have to make an ILogger that represents a lazy set of providers", which would be a bunch of work, but turns out we already almost had that!

[ryzngard]

Yeah, maybe it couldn't be done. At least, I can't visualize how we'd make output window logger track the log level. I suppose we could make deeper changes to make the runtime log part of the core logger infrastructure.

I wouldn't even be opposed to having specialized interfaces for IInformationLoggerProvider or something that could just lazy import whenever the log level meets the requirements. It's a bit verbose in a way but may be worth it

[DustinCampbell]

I wouldn't even be opposed to having specialized interfaces for IInformationLoggerProvider or something that could just lazy import whenever the log level meets the requirements. It's a bit verbose in a way but may be worth it

I went ahead and added support for exported ILoggerProviders to include a minimum log level. This is used to avoid instantiating an ILoggerProvider until a message is logged with an appropriate log level. At the moment, this just helps avoid instantiating the ActivityLogLoggerProvider, but it could be used for other things. Future work could add another bit to the MEF metadata that allows a provider to avoid being instantiated until a message is logged that is meets the Razor log level settings.

[DustinCampbell]

@dotnet/razor-compiler: You shouldn't need to review this PR. The only compiler relevant change I made is to remove an unused method before realizing that it was in the shared library. 😄

[333fred]

Shared code LGTM.