feat: flagd OTEL collector

[Kavindu-Dodan]

This PR

Fixes #563

Introduce configurations for flagd to connect to OTEL collector. Also, improves how telemetry configurations are handled.

Changes include,

• Package renaming - otel to telemetry - This avoids import conflicts

• Introduce a telemetry builder - The intention is to have a central location to handle telemetry configurations and build telemetry components

• Introduce a span processor builder - Provide groundwork for OTel Traces for evaluation in flagd #575 (needs fallback mechanism)

How to test?

Consider following this guide - (doc generated from commit) https://github.com/open-feature/flagd/blob/81c66b3c89540b475fe0a46ac89869800f7b74ae/docs/configuration/flagd_telemetry.md

In short,

• create configuration files (docker-compose yaml, collector config yaml, Prometheus yaml )
• start collector setup (docker-compose up)
• start flagd with otel collector override for metrics (flagd start --uri file:/flags.json --metrics-exporter otel --otel-collector-target localhost:4317)

Metrics will be available at Prometheus(http://localhost:9090/graph). Traces are still missing as we have to implement them.

[codecov]

Codecov Report

Merging #586 (b7c1123) into main (d78a3d9) will decrease coverage by 0.15%.
The diff coverage is 65.21%.

@@            Coverage Diff             @@
##             main     #586      +/-   ##
==========================================
- Coverage   70.91%   70.77%   -0.15%     
==========================================
  Files          21       22       +1     
  Lines        2187     2248      +61     
==========================================
+ Hits         1551     1591      +40     
- Misses        577      592      +15     
- Partials       59       65       +6     

Impacted Files	Coverage Δ
core/pkg/runtime/from_config.go	58.62% <0.00%> (-1.03%)	⬇️
...ore/pkg/service/flag-evaluation/connect_service.go	66.17% <ø> (ø)
core/pkg/service/flag-evaluation/flag_evaluator.go	68.34% <ø> (ø)
...ore/pkg/service/middleware/metrics/http_metrics.go	76.25% <ø> (ø)
core/pkg/telemetry/builder.go	68.42% <68.42%> (ø)
core/pkg/telemetry/metrics.go	100.00% <100.00%> (ø)

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[toddbaert]

I'm not an OTel expert, but this looks fine to me and I trust @bacherfl and @thisthat 's expertise.