Centralised one-shot exporter initialisation.

Every Trace SpanProcessor, Logs LogRecordProcessor, and Metrics MetricReader threads its user-supplied exporter through this helper at GenServer / gen_statem init/1 time so that:

• the exporter's own init/1 runs exactly once at startup;
• {:ok, state} becomes {module, state} and is stored as the per-call exporter handle;
• :ignore becomes nil, normalising the "no active exporter" case for the rest of the processor's code paths.

Mirrors erlang's otel_exporter:init/1 in references/opentelemetry-erlang/apps/opentelemetry/src/otel_exporter.erl. Without a single shared point, each processor / reader was duplicating the same case ... do {:ok, state} -> ...; :ignore -> nil end shape — and MetricReader.PeriodicExporting silently skipped the init/1 call entirely until the e2e smoke test in #392 surfaced it (see this commit's parent fix).