feat: add opentelemetry example for alloy

[sunng87]

This patch reuse alloy example to demo opentelemetry data ingestion.

For more information, see the README.

[daviderli614]

I tried to run the demo and got an error：

git clone https://github.com/GreptimeTeam/demo-scene.git
cd demo-scene/grafana-alloy

docker compose up
services.envsubst.env_file.0 must be a string

[sunng87]

@daviderli614 according to https://docs.docker.com/compose/how-tos/environment-variables/set-environment-variables/#additional-information-1, it requires a minimal version of docker compose. I will add to readme.

[sunng87]

@dedlockdave I was going to invite David but Li Yang's id was quite similar @daviderli614. Thank you for reviewing this and it has been quite helpful.