Map faas.* attributes to generic_task in resource mapping

[dashpole]

Fixes #272

Question: should faas.* attributes take precedence over service.* attributes if they are present? If they do, it might break users who are currently using service.* attributes. If we don't, the default service (unknown) will take precedence over faas.name

[dashpole]

Ideally: If a user sets a service.name, use that. Otherwise, fall back to faas.name.

Options:

1. resource detector detects service.name as faas.name. (would this cause schema URL conflicts?)
2. Check for serivce.name first, then fall back to faas.name. But ignore the unknown service name.
3. Faas.* takes precedence over service.name (this PR)

[dashpole]

I've proposed option 2 in a design document.

[dashpole]

I think this is good to go @aabmass.

[aabmass]

Thanks!