[3.5] Backport add sampling rate to fix distributed tracing

[jmhbnz]

ExperimentalDistributedTracingSamplingRatePerMillion is the number of samples to collect per million spans.
Defaults to 0.

Without this flag tracing is effectively broken / unusable in release-3.5 as the default sample rate was changed to 0 in 3.5.5 via 4c013c9.

Backports:

• server: Add sampling rate to distributed tracing #13248
• embed: add integration test for distributed tracing #14348

Fixes #16935

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[jmhbnz]

/test all

[jmhbnz]

Ready for review. With this branch we can once more see tracing spans in jaeger for etcd 3.5:

For reviewers, the testing procedure can be:

# Clone and compile pr branch
git clone  -b fix-distributed-tracing https://github.com/jmhbnz/etcd.git
cd etcd && make build

# Start jaeger instance
docker run -d --name jaeger   -e COLLECTOR_OTLP_ENABLED=true   -e COLLECTOR_ZIPKIN_HOST_PORT=:9411   -p 5775:5775/udp   -p 6831:6831/udp   -p 6832:6832/udp   -p 5778:5778   -p 16686:16686   -p 14250:14250   -p 14268:14268   -p 14269:14269   -p 4317:4317   -p 4318:4318   -p 9411:9411   jaegertracing/all-in-one:1.51

# Start etcd instance
./bin/etcd --experimental-enable-distributed-tracing=true --experimental-distributed-tracing-address=localhost:4317 --experimental-distributed-tracing-service-name=etcd --experimental-distributed-tracing-instance-id=hello --experimental-distributed-tracing-sampling-rate=999999&

# Generate some spans
sleep 10s
./bin/etcdctl put foo bar
./bin/etcdctl get foo
sleep 10s

# Open jaeger ui in browser and select etcd in dropdown
echo "http://localhost:16686"

# Clean up etcd and jaeger instances started earlier
# killall etcd
# docker rm --force jaeger

[jmhbnz]

cc @dashpole

[ahrtr]

lgtm

Thanks @jmhbnz

Please also add a changelog item before we merge this PR, in case we miss it.