Downgrade opentelemetry to v0.29 again in dora-metrics (#1132)

Version `v0.4.3` of `opentelemetry-system-metrics`, which updated to
`opentelemetry v0.30`, was yanked (as it violated semver). This causes
some build errors when dataflows/nodes are created through `dora new`
because those then use `opentelemetry-system-metrics v0.4.2` again,
which requires `opentelemetry v0.29`.

This PR fixes this issue by downgrading the `opentelemetry` dependencies
to v0.29 again (until `opentelemetry-system-metrics v0.5` is published).

Fixes #1110

Author: phil-opp