feat: Add opt-in OpenTelemetry auto-instrumentation to Docker images (#4281)

# What does this PR do?

This allows llama-stack users of the Docker image to use OpenTelemetry
like previous versions.

#4127 migrated to automatic instrumentation, but unless we add those
libraries to the image, everyone needs to build a custom image to enable
otel. Also, unless we establish a convention for enabling it, users who
formerly just set config now need to override the entrypoint.

This PR bootstraps OTEL packages, so they are available (only +10MB). It
also prefixes `llama stack run` with `opentelemetry-instrument` when any
`OTEL_*` environment variable is set.

The result is implicit tracing like before, where you don't need a
custom image to use traces or metrics.

## Test Plan

```bash
# Build image
docker build -f containers/Containerfile \
  --build-arg DISTRO_NAME=starter \
  --build-arg INSTALL_MODE=editable \
  --tag llamastack/distribution-starter:otel-test .

# Run with OTEL env to implicitly use `opentelemetry-instrument`. The
# Settings below ensure inbound traces are honored, but no
# "junk traces" like SQL connects are created.
docker run -p 8321:8321 \
  -e OTEL_EXPORTER_OTLP_ENDPOINT=http://host.docker.internal:4318 \
  -e OTEL_SERVICE_NAME=llama-stack \
  -e OTEL_TRACES_SAMPLER=parentbased_traceidratio \
  -e OTEL_TRACES_SAMPLER_ARG=0.0 \
  llamastack/distribution-starter:otel-test
```

Ran a sample flight search agent which is instrumented on the client
side. This and llama-stack target
[otel-tui](https://github.com/ymtdzzz/otel-tui) I verified no root
database spans, yet database spans are attached to incoming traces.


<img width="1608" height="742" alt="screenshot"
src="https://github.com/user-attachments/assets/69f59b74-3054-42cd-947d-a6c0d9472a7c"
/>

Signed-off-by: Adrian Cole <adrian@tetrate.io>

containers/Containerfile

@@ -120,6 +120,11 @@ RUN set -eux; \
         printf '%s\n' "$deps" | xargs -L1 uv pip install --no-cache; \
     fi
 
+# Install OpenTelemetry auto-instrumentation support
+RUN set -eux; \
+    pip install --no-cache opentelemetry-distro opentelemetry-exporter-otlp; \
+    opentelemetry-bootstrap -a install
+
 # Cleanup
 RUN set -eux; \
     pip uninstall -y uv; \
@@ -135,15 +140,21 @@ RUN cat <<'EOF' >/usr/local/bin/llama-stack-entrypoint.sh
 #!/bin/sh
 set -e
 
+# Enable OpenTelemetry auto-instrumentation if any OTEL_* variable is set
+CMD_PREFIX=""
+if env | grep -q '^OTEL_'; then
+  CMD_PREFIX="opentelemetry-instrument"
+fi
+
 if [ -n "$RUN_CONFIG_PATH" ] && [ -f "$RUN_CONFIG_PATH" ]; then
-  exec llama stack run "$RUN_CONFIG_PATH" "$@"
+  exec $CMD_PREFIX llama stack run "$RUN_CONFIG_PATH" "$@"
 fi
 
 if [ -n "$DISTRO_NAME" ]; then
-  exec llama stack run "$DISTRO_NAME" "$@"
+  exec $CMD_PREFIX llama stack run "$DISTRO_NAME" "$@"
 fi
 
-exec llama stack run "$@"
+exec $CMD_PREFIX llama stack run "$@"
 EOF
 RUN chmod +x /usr/local/bin/llama-stack-entrypoint.sh