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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`. 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