Preserve span table across otel_span_ets crashes

[QuinnWilton]

The span ETS table is created without {heir, Pid, Data}. When the
otel_span_ets process crashes, the table is destroyed and all in-flight
spans are silently lost. The process restarts and creates a fresh empty
table, but spans that were active between crash and restart are gone.
The try/catch in storage_insert/1 prevents cascading badarg errors but
masks the data loss.

Use the supervisor (otel_span_sup) as heir. On crash, ownership
transfers atomically to the supervisor. The restarted otel_span_ets
sees the table already exists via ets:info/2, skips ets:new, and
resumes operating on the preserved data. The table is public, so all
read/write operations continue to work regardless of which process
owns it.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: QuinnWilton / name: Quinn Wilton (5a7956a)

[QuinnWilton]

I originally had another commit in here for enabling read_concurrency on the table, but after running some benchmarks, it wasn't a clear win and I opted to remove that change and keep things simple.