roachprod: automate metrics export via datadog

Currently, datadog export automation is enabled only for drt clusters [1]. While both `opentelemetry-start` and `fluent-bit-start` are already exposed via `roachprod`, their configuration, including (datadog) secret management would have to be done out-of-band. Thus, wiring everything together into something like `--enable-datadog` would simplify things. (Also see the internal slack thread [2].)

[1] https://github.com/cockroachdb/cockroach/blob/master/pkg/cmd/drtprod/scripts/setup_datadog_cluster
[2] https://cockroachlabs.slack.com/archives/C023S0V4YEB/p1768612492946639?thread_ts=1768504502.738039&cid=C023S0V4YEB

Jira issue: CRDB-58888