Prepare future releases with breaking changes.

Due to expected changes in the spec, there will be breaking changes in the API/ABI.

Due to changes in the implementation, there are also known changes in the SDK interface, which can break applications when using shared libraries.

It is desirable to group many fixes that causes breaking changes at once, so listing here all the known issues that impact API/ABI.

It is also desirable to define the overall strategy to introduce breaking changes.

API/ABI:

• [Metrics API/SDK] Add InstrumentationScope attributes in MeterProvider::GetMeter() #2033 (API + SDK)
• [Trace API/SDK] Add InstrumentationScope attributes in TracerProvider::GetTracer() #2032 (API + SDK)
• Separate TextMapCarrier into Injector and Extractor interfaces #1922 (API)
• Allow creation of AttributeValue from nostd::span<const char*> #1416 (API)
• Can the API avoid returning shared_ptr<Span>  #828 (API) -> Providing alternate Span creation API to return nostd::unique_ptr.
• [Metrics API] The Record method of class Histogram now must have a context param, but in the Add method of class Counter it is optional #2155
• [Metrics API/SDK] - Support multiple Instrument callbacks #1753
• [Metrics API] Add support for histogram advice API #2132
• Add Synchronous Gauge instrument to the metrics API/SDK #2279

SDK:

• [EXPORTER] OTLP/GRPC Exporter options for metrics/logs do not honor _METRICS_/_LOGS_ environment variables. #1845 (remove exporter options struct inheritance)
• [Trace SDK] OTLP Trace Exporter secure connection configuration options #1402 (new members in exporter options struct)