feat(usage): rename metrics → tokens_embedded/pages_embedded + export token cost to Prometheus

Billing product model finalized (Deck #281): bill pages externally, record
tokens internally. Rename the data-plane metric literals to match the now-
canonical contract (Deck #284) — the control plane's METRIC_EVENT_NAMES is
already renamed, so the old names would be unmapped and never sync to Stripe.

Rename (values unchanged):
- embeddings_queries → tokens_embedded (value = real token count, already
  emitted by this PR; the unit upstream providers bill on).
- pages_chunks → pages_embedded (value kept as len(chunk_texts) interim;
  TODO(#282): real normalized "pages indexed" count — real pages for paginated
  types, chars/tokens-per-page constant otherwise — is deferred to the
  instrumentation card, this only lands the name/contract).
- All literals, log strings, docstrings, comments, the migration comment, and
  tests renamed; grep confirms zero old strings remain.

Observability (new): export embedding token cost to Prometheus as
astrolabe_embedding_tokens_total{provider,operation} (operation = index|query)
so the billed cost unit is visible in Grafana, not just the per-tenant billing
DB. Dedicated counter (doesn't inflate the existing chunk/request metrics) and
always-on (independent of USAGE_METERING_ENABLED, so OSS/self-host gets it).
Wired on both the indexing batch embed and the search query embed (query inside
the per-request cache-miss branch, so reused embeddings aren't double-counted).

Note: the rename orphans any pre-existing embeddings_queries/pages_chunks rows
in tenant app DBs (CP no longer maps them) — acceptable; pipeline is inert with
throwaway dev/sandbox data.

Deck #284 (folded into PR #875).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/observability/metrics.py

@@ -338,6 +338,16 @@ embedding_chars_total = Counter(
     ["kind", "provider"],
 )
 
+# Token consumption — the billed cost unit (mirrors the tokens_embedded billing
+# measure, Deck #67). On a dedicated counter (not folded into the chunk/request
+# metrics above) so query embeds don't inflate indexing dashboards; labelled by
+# operation = index | query. Always emitted, independent of USAGE_METERING_ENABLED.
+embedding_tokens_total = Counter(
+    "astrolabe_embedding_tokens_total",
+    "Total embedding tokens consumed (provider-reported or estimated)",
+    ["provider", "operation"],  # operation: index | query
+)
+
 # --- Chunking & indexed-by-type -----------------------------------------------
 
 document_chunks_total = Counter(
@@ -727,6 +737,25 @@ def record_embedding(
             embedding_chars_total.labels(kind=kind, provider=provider).inc(chars)
 
 
+def record_embedding_tokens(provider: str, operation: str, tokens: int) -> None:
+    """Export embedding token consumption to Prometheus.
+
+    Mirrors the ``tokens_embedded`` billing measure (Deck #67) as an always-on
+    observability signal — emitted regardless of ``USAGE_METERING_ENABLED`` so
+    OSS/self-host deployments still see token cost in Grafana.
+
+    Args:
+        provider: Provider family (mistral | openai | bedrock | ollama | simple).
+        operation: ``"index"`` (chunk-batch embedding) or ``"query"`` (search
+            query embedding).
+        tokens: Token count for this embedding request (no-op when ``<= 0``).
+    """
+    if tokens > 0:
+        embedding_tokens_total.labels(provider=provider, operation=operation).inc(
+            tokens
+        )
+
+
 def record_document_chunks(doc_type: str, count: int) -> None:
     """
     Record the number of chunks produced for a document.