instance attribute support for charm_tracing forward-declared definitions

[PietroPasotti]

This PR adds support for charm_tracing to grab the endpoint and cert it needs from instance attributes instead of properties and methods.

This is a nicety which in combination with #135 means any charm can enable/disable charm tracing and correctly configure it with/without TLS with a single line of code (plus the decorator).

before:

from lib....charm_tracing import charm_tracing

@charm_tracing(tracing_endpoint="foo", server_cert="bar")
class MyCharm(...):
    [...]
    def foo(self):
        # complicated logic to return None if we don't have tracing or we have tracing and tls but no cert yet
        return ... 
    def bar(self):
        return ...

after:

from lib....charm_tracing import charm_tracing
from lib....tracing import charm_tracing_config

@charm_tracing(tracing_endpoint="foo", server_cert="bar")
class MyCharm(...):
    def __init__(...):
        self.tracing = TracingEndpointRequirer(...)
        self.foo, self.bar = charm_tracing_config(self.tracing, "path/to/cert/")

Additionally:

• tuned down logs
• be less forgiving and raise if tracing_endpoint returns rubbish or raises, like we're doing in charm_logging

[mmkay]

Muuuuch nicer, thanks @ca-scribner!