Cleanup opencensus shim

[jack-berg]

• Delete deprecated OpenTelemetryMetricExporter
• Make all classes final
• Merge io.opentelemetry.opencensusshim.internal.metrics into io.opentelemetry.opencensusshim
• Make classes package private that aren't part of public API

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Files	Coverage Δ
...io/opentelemetry/opencensusshim/MetricAdapter.java	88.17% <ø> (ø)
...metry/opencensusshim/OpenCensusMetricProducer.java	100.00% <100.00%> (ø)
...ry/opencensusshim/OpenTelemetryContextManager.java	77.77% <ø> (ø)
...opentelemetry/opencensusshim/OpenTelemetryCtx.java	90.00% <ø> (ø)
...ensusshim/OpenTelemetryNoRecordEventsSpanImpl.java	80.00% <ø> (ø)
...susshim/OpenTelemetryPropagationComponentImpl.java	100.00% <100.00%> (ø)
...y/opencensusshim/OpenTelemetrySpanBuilderImpl.java	87.17% <100.00%> (ø)
...ry/opencensusshim/OpenTelemetryTextFormatImpl.java	87.50% <ø> (ø)
...pencensusshim/OpenTelemetryTraceComponentImpl.java	88.23% <ø> (ø)
...emetry/opencensusshim/OpenTelemetryTracerImpl.java	100.00% <100.00%> (ø)
... and 1 more

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