## Motivation

Support the latest OpenTelemetry specification.

## Solution

Update `opentelemetry` to the latest `0.18.x` release. Breaking changes
in the metrics spec have removed value recorders and added histograms so
the metrics layer's `value.` prefix has been changed to `histogram.` and
behaves accordingly. Additionally the `PushController` configuration for
the metrics layer has been simplified to accept a `BasicController` that
can act in either push or pull modes. Finally trace sampling in the
sdk's `PreSampledTracer` impl has been updated to match the sampling
logic in open-telemetry/opentelemetry-rust#839.