[WIP] Metadata providers

[slashmo]

Add MetadataProvider closure to be run on each log statement to attach the current value of something (e.g. a task-local) to the statement's metadata values (#205).

Motivation:

Closes #205

Modifications:

• LoggingSystem gains a new provideMetadata method
• LoggingSystem stores an array of MetadataProviders
• Logger gains a new initializer taking an overwriting MetadataProvider
• Logger executes the MetadataProvider before logging a statement, attaching the resulting values to the statement's metadata

Result:

It becomes easier to attach metadata to each log statement where the value can be derived without a method argument such as retrieving from the current task local storage. Instead of writing the code to retrieve such values per log statement it can instead be set up once.

[ktoso]

@swift-server-bot test this please

[slashmo]

This PR is now superseded by https://github.com/apple/swift-log/pull/235