telemetry/gnmi-writer: store interface packet-class counters

[snormore]

Summary of Changes

• Persist per-interface unicast, multicast, and broadcast packet counters (in/out) plus FCS errors to the interface_state table — these already stream in the gNMI interface-state subtree alongside the octet/packet counters we keep, so no upstream subscription change is needed.
• Brings the ClickHouse interface_state table closer to parity with the interface counters already collected in InfluxDB.
• Add the seven new columns to interface_state via a ClickHouse migration (ADD COLUMN is metadata-only; existing rows read back 0) and recreate the interface_state_latest view so SELECT * surfaces them.
• Unblocks downstream rollups (e.g. device_interface_rollup_5m) that consume in_fcs_errors and multicast PPS metrics.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Tests	2	+96 / -2	+94
Scaffolding	2	+63 / -0	+63
Core logic	1	+21 / -0	+21
Total	5	+180 / -2	+178

Small, mechanical change: the bulk is tests and a schema migration; the actual logic is seven nil-checked field copies in the extractor.

Key files (click to expand)

• telemetry/migrations/20260520000000_interface_state_packet_counters.sql — new migration adding the 7 UInt64 columns and recreating the _latest view (with full down-migration)
• telemetry/gnmi-writer/internal/gnmi/processor_test.go — unit test asserting all 16 counters map to the correct record fields (distinct values catch column swaps)
• telemetry/gnmi-writer/internal/gnmi/processor_integration_test.go — integration test asserting the 7 new columns round-trip through ClickHouse on an active interface with real traffic
• telemetry/gnmi-writer/internal/gnmi/extractors.go — populate the new counter fields from iface.State.Counters.*
• telemetry/gnmi-writer/internal/gnmi/records.go — add the 7 fields to InterfaceStateRecord with ch: column tags

Testing Verification

• Unit test (TestExtractInterfaceState_Counters): builds an oc.Device with distinct non-zero values for all 16 counters and asserts the full extracted InterfaceStateRecord, guarding against field-assignment mistakes.
• Integration test (verifyInterfaceState): validates the gNMI fixture → extractor → ClickHouse INSERT → query-back path for the new columns against a real ClickHouse container, asserting non-zero values on Switch1/11/2 — this exercises the ch: tag and migration column-name mapping end to end.
• Full TestIntegration suite passes against a containerized ClickHouse with all migrations applied.