Add user metadata to S3 objects

[vcschapp]

Is your feature request related to a problem? Please describe.

I need to deliver objects to S3 that have a specific user metadata tag, i.e. Foo=Bar or in this case Format=Baz.

Describe the solution you'd like

1. Add a new user_metadata configuration parameter to the Amazon S3 output plugin where the type of values for parameter must be a map of string -> string.
2. In the S3 PutObject or CreateMultipartUpload API call that the plugin uses to create the object in S3, add the user_metadata tags, if any are specified.

Initially these tags would be specified statically in the config and would not have any dynamic content.

Describe alternatives you've considered

I need my objects to have a specific metadata key for compatibility with an existing web service I do not control. My only options are:

1. Don't use FB.
2. Write a new custom plugin specifically for my use case that does what I want.
3. Make the existing Amazon S3 plugin more versatile.

I do think this feature would be broadly used and useful.

Additional context

• I am trying to send usage logs to an existing web service whose "public interface" is an S3 bucket that MUST receive objects that have a specific metadata key/value pair identifying the format of the contents.