[Telemetry] use existed env variable instead of introducing new one

[louie-tsai]

Description

Remove new environment variable and use existed one to disable/enable telemetry feature
PR#1168

Issues

NA

Type of change

List the type of change like below. Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would break existing design and interface)
• Others (enhancement, documentation, validation, etc.)

Dependencies

no

Tests

manually testing

[eero-t]

Approved, matches env var used in telemetry code:

GenAIComps/comps$ git grep TELEMETRY | grep py:
__init__.py:if os.getenv("ENABLE_OPEA_TELEMETRY", "false").lower() == "true":
cores/telemetry/opea_telemetry.py:telemetry_endpoint = os.environ.get("TELEMETRY_ENDPOINT", "http://localhost:4318/v1/traces")

[lianhao]

Instead of enabling/disabling the tracing in comps/init.py, could we do this in the module comps.cores.telemetry.opea_telemetry.opea_telemetry itself.

Doing a simple grep, I found other places just directly import the opea_telemetry.

$ grep -r 'import opea_telemetry' `find -name "*.py"`
./comps/__init__.py:    from comps.cores.telemetry.opea_telemetry import opea_telemetry
./comps/rerankings/src/opea_reranking_microservice.py:from comps.cores.telemetry.opea_telemetry import opea_telemetry
./comps/embeddings/src/opea_embedding_microservice.py:from comps.cores.telemetry.opea_telemetry import opea_telemetry
./comps/cores/mega/orchestrator.py:from ..telemetry.opea_telemetry import opea_telemetry, tracer
./comps/llms/src/text-generation/opea_llm_microservice.py:from comps.cores.telemetry.opea_telemetry import opea_telemetry