Improve automatic configuration

[djmitche]

Ideally in most cases, users of this library would not pass host/port or socket to the constructor, allowing configuration by environment variables and in their absence, selecting an appropriate default.

DataDog/dd-trace-rb#1700 did something similar for dd-trace-rb, selecting a socket if the expected file exists.

[djmitche]

This would need some tracking across libraries/languages (AC-1367)