Add sdk/telemetry package

[MrAlias]

This package is the replacement for pdata within the sdk package.

It is designed to be ...

• Have minimal dependencies outside of the standard library (or anything the otel package already depends on)
• Be compatible with OTLP's protobuf JSON encoding
  • This means it will provide a forwards compatible binary encoding that users can rely on
• Have a reasonably user friendly Go API
• Reasonably performant

It is opinionated, and designed to work best for our needs.

Part of #954
Part of #1106
Split from #1126

Follow up work

• Add sdk/telemetry/test pacakge to test the compatibility with pdata
• Update the sdk package to use telemetry as its data-model

[RonFed]

What do you think about renaming the package to something like: sdk/encoding, sdk/datamodel or sdk/data?

[MrAlias]

What do you think about renaming the package to something like: sdk/encoding, sdk/datamodel or sdk/data?

I am trying to stay away from things like data or datamodel as they are overly generic and don't explicate what they contain explicitly and are apt for name collisions with other "data-models".

Naming the package encoding was considered. However, the types themselves do not represent the actual encoding, and if it is interpreted as a verb, this would only cover part of the provided functionality (it also supports decoding). This also suffers from the "overly generic" naming issue.

I preferred using a package name that describes uniquely and clearly what the package contains. I'm open to other names, but would like something that describes telemetry data that is going to be represented and interacted with.