Add withMetadata() convenience function for adding multiple metadata k/v pairs to a Logger

[gabbifish]

Adds withMetadata() convenience function for adding multiple metadata k/v pairs to a Logger.

Motivation:

Addresses #321

Modifications:

Note that in the spirit of the Logger[metadataKey: key] = value subscript mutating a logger, this PR also mutates the logger in-place.

Result:

Provides users a convenience function to add or modify multiple Logger.Metadata values in a Logger in place.