Request to improve Instrumentation for ALL

[bogdandrutu]

A common pattern in instrumentations is to retrieve a Tracer almost every time a span is created, this is not the pattern we want to encourage, and we should change this ASAP to avoid other copying this pattern.

See some examples:

• Avoid getting a new Tracer for every RPC #2835 - fixed
• https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/instrumentation/github.com/Shopify/sarama/otelsarama/example/consumer/consumer.go#L96 - here the tracer can be global var (if we use the global provider) or a member of Consumer (if we want to pass a Provider when constructing a Consumer).

The person who is going to work on this should find all these patterns and fix.

[Aneurysm9]

I think there's a third way that should be what we recommend. The net/http instrumentation will use a single Tracer instance if a TracerProvider was provided at configuration time, but otherwise will attempt to extract a TracerProvider from the active span and then fall back to the global TracerProvider if there is no active span. This allows for simplified use in multi-tenant scenarios where multiple TracerProviders may be in use while allowing those without that requirement to ensure only one Tracer is ever constructed.

[bogdandrutu]

If the fallback to global TraceProvider, also we should optimize to not call global.TraceProvider().Get(...) every request. You may actually want to provide a mini library for all the logic.