change extras method of MetadataSourceHandler to be async

We should make this extras method an async method, so that it can be used to call other async functions/methods:

biothings.api/biothings/web/handlers/query.py

Lines 155 to 159 in d3a001a

    
               def extras(self, _meta): 
        
                   """ 
        
                   Override to add app specific metadata. 
        
                   """ 
        
                   return _meta

For use cases without async calls, it can be a simple yield to return updated _meta value.

We should check other API instances override extras and make the change to work with this new change.

Here is a use case in MyGene.info repo:

https://github.com/biothings/mygene.info/blob/409c1d812dac4218c042c02d30b8fe87912882c2/src/web/handlers.py#L65-L76

Another place we override extras:

https://github.com/NIAID-Data-Ecosystem/nde-discovery-api/blob/69c0048afe6688b5d95877c87f4a28bac27496c3/nde-web/handlers.py#L19

released in 0.12.0

	def extras(self, _meta):
	"""
	Override to add app specific metadata.
	"""
	return _meta