docs: fix the HTTP and gRPC transports quickstart guides

[Henauxg]

Changes

Fixed the opentelemtry-otlp quickstart guides for HTTP & gRPC

HTTP:

• Added missing call to global::set_tracer_provider

gRPC:

• Fixed the main sample to use [tokio:main] and to call global::set_tracer_provider
• Added a small sample using gRPC without [tokio:main] (based on opentelemtry-otlp/examples/basic-otlp)

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (NA)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes (NA)
• Changes in public API reviewed (NA)

Tests

cargo test --all fails on test build_tonic but this is also the case on main

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.3%. Comparing base (9cdc931) to head (3c466f2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2933     +/-   ##
=======================================
- Coverage   81.3%   81.3%   -0.1%     
=======================================
  Files        126     126             
  Lines      24254   24254             
=======================================
- Hits       19736   19733      -3     
- Misses      4518    4521      +3     

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[cijothomas]

Thanks!
If you have more bandwidth, please continue improving this doc in future PRs.
Some missing things:

1. Use batch_processor instead of simple. (or mention in code comment about batch being the production recommendation)
2. Show tracerProvider shutdown at the end.

(Then extend the doc to logs, metrics)