Support OpenTelemetry export from gateways - #4467

Gateways can now export their traces, logs, and
metrics via OTLP over HTTP. Configure it with the
new `otel` property when creating a gateway:

```
type: gateway
name: example-gateway

backend: aws
region: eu-west-1

domain: example.com

otel:
  endpoint: https://otlp.example.com:4318
  headers:
    Authorization: Bearer ${{ secrets.otlp_token }}
  traces: true
  logs: true
  metrics: true
```

`endpoint` and `headers` can reference project
secrets. The `otel` configuration is visible to
everyone who can view the gateway, including
importer projects, so credentials should be stored
in secrets.

In-place update for the `otel` property is not
supported yet.