Log `openTelemetryMode` in debug logs

### Description

See https://github.com/getsentry/sentry-java/pull/4091#discussion_r1926852015

We want to log something at startup so it's easier to help customers when they report issues.

It could look like:
> `openTelemetryMode` is set to `AGENTLESS`

or

> `openTelemetryMode` is set to `AUTO`
> Inferred `OpenTelemetryMode.AGENTLESS` (was set to AUTO) as our `AgentlessMarker` was detected