Silent-failure ODS counters (#5850)

[adityas-meta]

Summary:

X-link: https://github.com/facebookresearch/FBGEMM/pull/2768

Add OBC counters for 3 silent failure modes in the Raw Embedding Streaming (RES) path, closing the must-have bar of T273802725 (subtask of T269497764, RES code-side observability this half). Today these failure sites only emit XLOG(ERR) / LOG(ERROR) — nothing aggregates, so on-call cannot trigger off them or query them in ODS without tail -f on individual trainer hosts.

Counters added (all via TrainingPsOdsLogger::bumpKey, OBC category raw_embedding_streaming):

Counter	Site
res.fail.shard_size_mismatch	raw_embedding_streamer.cpp — both the outer indices.size(0) != weights.size(0) guard and the per-shard weights_masked.size(0) != rows_in_shard guard in tensor_stream()
res.fail.set_embeddings_rpc	raw_embedding_streamer.cpp — wraps co_await res_client->co_setEmbeddings(req) in try/catch; bumps then re-throws to preserve trainer-thread termination
res.fail.write_sparse_throw	TrainingPsHandler.cpp catch (const std::exception& e) block at the writeSparseListOfTensors site in co_stream_tensors

Backend (addresses review: use OBC, not fb303): all three route through the existing TrainingPsOdsLogger OBC client. The handler already owned an ods_logger_; the fbgemm-side RawEmbeddingStreamer now holds a TrainingPsOdsLogger member (forward-declared in the OSS-mirrored header, constructed only when streaming is enabled). OBC reaches ODS through the host-level agent with no per-process fb303 export/scrape config, so this revision deletes the fb303 ResFailCounters.h helper (and its call_once / addStatExportType / FOLLY_EXPORT machinery) entirely.

Counters bump AT the failure site (inside catch / right before continue) so they reflect what actually happened — applying the correctness lesson from D104076551 V4 (don't preemptively count attempts at the top of the function). Per-TBE-per-rank granularity is fine: OBC aggregates per (entity, key), so per-TBE bumps land on the same per-host key rather than separate rows.

Existing log lines preserved (additive). Existing throw-on-failure semantics preserved — the co_setEmbeddings catch re-throws after bumping so trainer-thread termination is unchanged.

Out of scope (intentional, per T269497764 boundary):

• MPSC weights_to_stream_queue_ near-full counter — queue is unbounded; deferred to the queue-depth gauge work in T273802756.
• Detectors / alerts on these counters — downstream consumer work.

Differential Revision: D107811590