[Otel Hook] Add metrics hook to OpenFeature.Contrib.Hooks.Otel

[toddbaert]

We should add a metrics hook to OpenFeature.Contrib.Hooks.Otel.

Similar to other implementations, this should be a breaking change to make the pruposes of each hook clearer. OtelHook should be renamed to TracingHook, and a new MetricsHook should be added.

export const ACTIVE_COUNT_NAME = ${FEATURE_FLAG}.evaluation_active_count;
export const REQUESTS_TOTAL_NAME = ${FEATURE_FLAG}.evaluation_requests_total;
export const SUCCESS_TOTAL_NAME = ${FEATURE_FLAG}.evaluation_success_total;
export const ERROR_TOTAL_NAME = ${FEATURE_FLAG}.evaluation_error_total;

Metrics hook requirements:

• should add 4 metrics (feature_flag.evaluation_active_count, feature_flag.evaluation_requests_total, feature_flag.evaluation_success_total, feature_flag.evaluation_error_total), similar to the JS / Java / Go implementations

• support a functional means of adding custom attributes to metrics, as in the JS implementation, which allows consumers to provide a function that takes the flag metadata and returns OTel attributes

• Rename OtelHook into TracingHook #112

• Create a new MetricsHook #113

[askpt]

@toddbaert can we split this issue into two separate issues?

1. Rename OtelHook into TracingHook
2. Create a new MetricsHook

About point 1, should we keep a "dummy" OtelHook and mark it as deprecated and it will be removed in a later version? Or just rename and the consumers will break upon update?

[toddbaert]

@toddbaert can we split this issue into two separate issues?

1. Rename OtelHook into TracingHook
2. Create a new MetricsHook

About point 1, should we keep a "dummy" OtelHook and mark it as deprecated and it will be removed in a later version? Or just rename and the consumers will break upon update?

Yes to all of the above!

[askpt]

@toddbaert this can be closed since #118 exists now :)