OpenCensus bridge: GA improvements

[dashpole]

Problem Statement

Make sure we are happy with the package API before we consider making a stable release.

Current potential problems:

• NewTracer takes an OpenTelemetry Tracer, rather than an OpenTelemetry TracerProvider. Ideally, the bridge would always have the correct instrumentation scope, rather than relying on users to set it correctly.
• NewTracer and NewMetricProducer do not accept Options. We should consider adding this in-case we need options in the future.
• NewTracer needs to be assigned to the opencensus.DefaultTracer to function properly. It might be more helpful to users to do that for them.
• opencensus.OTelSpanContextToOC and opencensus.OCSpanContextToOTel names are hard to read. Possibly opencensus.SpanContextFromOpenTelemetry and opencensus.SpanContextToOpenTelemetry?
• Remove deprecated NewMetricExporter

[dashpole]

I think we should leave the OCSpanContextToOTel names as-is. Either way, the names are going to be long, and I don't think users will need to use either of those functions often.