Consolidate Observer and Counter example

[mayurkale22]

I think we should consolidate Observer and Counter example into one. It would be easier to find and comprehend to the end users.

Observer example: https://github.com/open-telemetry/opentelemetry-js/blob/master/examples/metrics/metrics/observer.js

Counter example: https://github.com/open-telemetry/opentelemetry-js/blob/master/examples/prometheus/index.js

[smith]

The "Observer example" linked above is a 404; it may of been moved but I'm pretty sure it now refers to https://github.com/open-telemetry/opentelemetry-js/blob/main/experimental/packages/opentelemetry-sdk-metrics-base/examples/metrics/metrics/observer.js.

[legendecas]

The observer API is still experimental. It is likely going to be improved before the GA of the metrics SDK. I'll keep track of this at #2574

[dyladan]

The observer example was removed in the rewrite. I'm closing this since I feel it is covered by #3026