Update example prom-statsd-exporter config

[jcdavis]

Few cleanups for folks using this as a baseline (aka myself):

1. Remove timer_type for counter metrics, this field is a no-op as-is and is just confusing.
2. Where it is needed (ratelimit_service_rate_limit_shadow_mode), switch to observer_type, since timer_type has been deprecated since at least 2020: https://github.com/prometheus/statsd_exporter/commit/4a64979563442b948c6b929daee10f3b15493681
3. Add a single-key shadow mode match (even if this is not necessary/used by the example ratelimit setup)

[github-actions[bot]]

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[ysawa0]

Looks great. @jcdavis Should we also update the image ver here? https://github.com/envoyproxy/ratelimit/blob/main/docker-compose-example.yml#L13

[ysawa0]

Please fix DCO