Inline OTLP metrics exporter

[shuheiktgw]

Summary

• Replace the metrics-exporter-otel dependency with a local OTLP metrics recorder implementation.
• Keep the existing OTLP SDK version and protocol/temporality config behavior unchanged.
• Move the metrics exporter tests into a dedicated module file.

Tests

• make fmt
• cargo test -p quickwit-telemetry-exporters --lib

[guilload]

What's the rationale for doing this?

[shuheiktgw]

@guilload Right now, metrics-rs doesn’t support OTLP exporters by default. There is an open PR for it, but I’m not sure when it will be merged.

In the previous PR, I used metrics-exporter-otel instead, but it doesn’t seem to be very widely used, so I’m concerned that relying on it could introduce supply chain risk. There is another similar crate, but that also doesn’t seem to be widely adopted.

The exporter implementation is relatively simple, so I was thinking it might be better for us to just write it ourselves. What do you think?