feat(tvix/store): add opentelemetry

This is behind the otlp feature flag (currently enabled by default).

By default, this will try to push traces to a OTLP collector running at
https://localhost:4317.

You can make one available by running:

```
docker run -d --name jaeger \
  -e COLLECTOR_ZIPKIN_HOST_PORT=:9411 \
  -e COLLECTOR_OTLP_ENABLED=true \
  -p 6831:6831/udp \
  -p 6832:6832/udp \
  -p 5778:5778 \
  -p 16686:16686 \
  -p 4317:4317 \
  -p 4318:4318 \
  -p 14250:14250 \
  -p 14268:14268 \
  -p 14269:14269 \
  -p 9411:9411 --rm \
  jaegertracing/all-in-one:1.42
```

Started like that, jaeger brings a web interface at
http://localhost:16686/search

As documented in
https://docs.rs/opentelemetry-otlp/latest/opentelemetry_otlp/, you can
point this to another location by setting `OTEL_EXPORTER_OTLP_ENDPOINT`.

Change-Id: Id1dca367d70027b2ea98bb70bcf99a68363ec2be
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8194
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: aaqaishtyaq <aaqaishtyaq@gmail.com>

tvix/default.nix

@@ -32,6 +32,10 @@ let
         buildInputs = prev.buildInputs or [ ] ++ iconvDarwinDep;
       };
 
+      opentelemetry-proto = prev: {
+        nativeBuildInputs = protobufDep prev;
+      };
+
       prost-build = prev: {
         nativeBuildInputs = protobufDep prev;
       };