Sort attribute tables by requirement level and attribute name

[pyohannes]

Fixes open-telemetry/semantic-conventions#614

When consulting semantic conventions and the documentation generated from it, especially for metrics, I often find myself asking which attributes I can rely on to be present (which ones are "required"). For me, that's a much more common use case than looking for a single attribute by name.

This changes the ordering of generated attribute tables to order by requirement level: required attributes first, then conditionally required ones, recommended ones, and opt-ins. Especially for metrics, looking for required attributes is a more common use case than looking attributes up by name.

Example

That's how this would look for http.server.request.duration attributes:

Attribute	Type	Description	Examples	Requirement Level
http.request.method	string	HTTP request method. [1]	GET; POST; HEAD	Required
url.scheme	string	The URI scheme component identifying the used protocol. [2]	http; https	Required
error.type	string	Describes a class of error the operation ended with. [3]	timeout; java.net.UnknownHostException; server_certificate_invalid; 500	Conditionally Required: If request has ended with an error.
http.response.status_code	int	HTTP response status code.	200	Conditionally Required: If and only if one was received/sent.
http.route	string	The matched route, that is, the path template in the format used by the respective server framework. [4]	/users/:userID?; {controller}/{action}/{id?}	Conditionally Required: If and only if it's available
network.protocol.name	string	OSI application layer or non-OSI equivalent. [5]	http; spdy	Conditionally Required: [6]
network.protocol.version	string	Version of the protocol specified in network.protocol.name. [7]	1.0; 1.1; 2; 3	Recommended
server.address	string	Name of the local HTTP server that received the request. [8]	example.com; 10.1.2.80; /tmp/my.sock	Opt-In
server.port	int	Port of the local HTTP server that received the request. [9]	80; 8080; 443	Opt-In

[lmolkova]

LGTM!

(please add a changelog entry)

[lmolkova]

There was a bunch of new merge conflicts - I resolved them. Keeping it open a bit longer in case anyone wants to take a look.

[pyohannes]

There was a bunch of new merge conflicts - I resolved them. Keeping it open a bit longer in case anyone wants to take a look.

@lmolkova Looks good to me.