Fix nil metrics panic in VSCC SQL state DB writes

[krishnanmano]

TL;DR

VSCC initializes its SQL state DB provider with a nil metrics provider (statesqldb.NewVersionedDBProvider(..., nil, nil)), so registerMetricsOnce.Do never runs and db.metrics stays nil. ApplyUpdates/GetLatestSavePoint then dereference it unconditionally, panicking on the first block commit on any channel using the standalone VSCC service. Passes the real metricsProvider through at the call site and guards every db.metrics dereference so a nil provider (e.g. the snapshot-import path, which intentionally passes nil) degrades gracefully instead of crashing.

Files to review (2, +25 / -13):

File	Why
core/vscc/validate.go (start here)	Root cause: NewVersionedDBProvider was called with nil instead of metricsProvider.
core/ledger/kvledger/txmgmt/statedb/statesqldb/statesqldb.go	Wraps every db.metrics.* call in ApplyUpdates and GetLatestSavePoint with a nil check.

Reviewer notes

• Both the root cause and the guard are fixed. ImportFromSnapshot calls provider.newVersionedDB(dbName, nil, nil) on purpose, so db.metrics can legitimately be nil outside VSCC too — the nil guards keep that path safe even though the VSCC call site is now correct.
• No behavior change on the happy path. When a metrics provider is present, metrics are recorded exactly as before; the guard only skips recording when db.metrics is nil.

Links

• Fixes Nil db.metrics panic on first SQL write via VSCC #7