fix: use gRPC otel exporters instead of HTTP

[simpsonw]

Description

This PR changes our otel exporter to use gRPC instead of HTTP.

Context / Why are we making this change?

We're getting errors like:

failed to upload metrics: Post "http://localhost:4317/v1/metrics": net/http: HTTP/1.x transport connection broken: malformed HTTP response

It looks like port 4317 is for gRPC rather than HTTP (open-telemetry/opentelemetry-specification#1816 (comment)), so I think we're effectively sending an HTTP request to a gRPC endpoint.

Testing and QA Plan

I built this locally and tested it in the ESP sandbox.

Impact

We'll use gRPC for otel exporting.

[simpsonw]

Closed in favor of #13