add proxy_url to otelcol.exporter.otlp components

[steventill]

Request

the otelcol.exporter.otlp & otelcol.exporter.otlphttp components do not have an individual proxy option. it appears that it is an undocumented feature open-telemetry/opentelemetry-java#6204

i was expecting something similar to the prometheus.remote_write client block. where you would do something like this:

otelcol.exporter.otlp "default" { client { endpoint="https://otlp.grafana.net" proxy_url="http://internal.proxy.com:1234" auth=otel.auth.basic.default } } otelcol.auth.basic "default" { user="username" password="password" }

Use case

the only way to do this now is to set as an extraEnv the HTTPS_PROXY, this is really unclear, adding this to the block would clean it up, and make it easier to use proxies. In addition, this would allow support for multiple proxies

[wildum]

I opened a PR to add the proxy_url to the otlphttp exporter. I think that this attribute is not available in the upstream component for grpc