[contribfest] use new `get*FromEnv()` functions in OTLP gRPC exporter env var configuration

[pichlermarc]

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Description

[!IMPORTANT] This issue is reserved for attendees of OpenTelemetry ContribFest at KubeCon EU 2025 If you are attending KubeCon EU 2025, please join us at Level 3 | ICC Capital Suite 1 on Wednesday April 2, 2025 16:15 - 17:30 BST 🙌

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With #5443 we've introduced new utility functions for getting getting environment variables in an OTel Spec compliant manner, and we've migrated from using the old getEnv() utility function. However, there are still some places which never used getEnv() at all as it was on it's way to being phased out, using process.env directly instead. The OTLP exporter configuration code is one of these places.

The goal of this issue is to replace usages of process.env.MY_ENV_VAR in the OTLP gRPC Exporter's configuration code (see here)

To complete this issue:

• [ ] check the implementation of the new utility functions
• [ ] use these new utility functions in the OTLP gRPC exporter's configuration code
• [ ] the new utility functions do already drop blank and nullish strings, please remove any now-unncessary checks
• [ ] ensure unit-tests are still passing