mock: differentiate between mocks and expectations

[hds]

Motivation

The tracing-mock crate provides a mock collector (and a subscriber for
use by the tests in the tracing-subscriber crate) which is able to
make assertions about what diagnostics are emitted.

These assertions are defined by structs that match on events, span, and
their fields and metadata. The structs that matched these objects have
been called, up until now, mocks, however this terminology may be
misleading, as the created objects don't mock anything.

There were two different names for similar functionality with only()
and done() on fields and collectors/subscribers respectively. Using a
single name for these may make it easier to onboard onto tracing-mock.

Solution

To reduce confusion, these structs have been split into two categories:
mocks and expectations.

Additionally, the done() function on the Collector and Subscriber
mocks has been replaced with only(). This matches the similar function
for ExpectedField, and may be more intuitive.

The mocks replace some component in the tracing ecosystem when a library
is under test. The expectations define the assertions we wish to make
about traces received by the mocks.

Mocks (per module):

• collector - MockCollector, no change
• subscriber - MockSubscriber, renamed from ExpectSubscriber

Expectations (per module):

• event - ExpectedEvent, renamed from MockEvent
• span - ExpectedSpan, renamed from MockSpan
• field - ExpectedField and ExpectedFields, renamed from MockField
  and Expected. Also ExpectedValue renamed from MockValue.
• metadata - ExpectedMetadata, renamed from Expected

Refs: #539

[davidbarsky]

this looks good to me overall—I like the changes.

quick question about .only() being used at the top, Collector level: is that only necessary to to ensure that that it's an exact match? if so, can that be elided entirely?

[hds]

The .only() (previously .done() for the MockCollector) ensures that no further traces are received after the specified ones. This is roughly equivalent to checking that there are no additional fields when only() is used on an ExpectedFields.

I don't think I understand the question about eliding it complete though.