telemetry: log and count samples dropped at buffer capacity

[elitegreg]

Resolves: #4126

Summary of Changes

• The telemetry submitter now logs a warning and increments counters when it discards samples because a submission failed and the partition buffer is already at capacity. That path previously recycled the batch silently, so lost measurement data left no trace beyond submitter_retries_exhausted, which fires whether the batch was requeued or destroyed.
• New counter doublezero_device_telemetry_agent_samples_dropped_total with a reason label (buffer_full), so the account-full drop path (device/telemetry: account-full path reports success and logs the wrong count #4127) can report on the same series with a different reason.
• New submitter_buffer_full error type on the existing doublezero_device_telemetry_agent_errors_total counter, one increment per dropped batch.
• Requeue behavior below capacity is unchanged. Neither signal fires in steady state: a partition holds 4096 samples, roughly 11 hours of backlog at 6 samples per minute.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Tests	1	+89 / -0	+89
Core logic	1	+14 / -5	+9
Scaffolding	1	+14 / -0	+14
Docs	1	+3 / -0	+3
Total	4	+120 / -5	+115

Mostly tests around a nine-line observability fix; no behavior change to submission or requeueing.

Key files (click to expand)

• controlplane/telemetry/internal/telemetry/submitter.go — the over-capacity branch in Tick now warns with the dropped count, buffer length, and capacity, and bumps both counters; the requeue branch is unchanged apart from being nested under the existing !success check
• controlplane/telemetry/internal/metrics/metrics.go — registers the samples_dropped_total counter, the reason label, and the submitter_buffer_full error type

Testing Verification

• New test fills a partition to exactly its capacity, fails every submission attempt, and asserts the warning is emitted with droppedSamples=2 and capacity=2, that the drop counter rises by the batch length, that the error counter rises once, and that nothing is requeued. Confirmed it fails against the pre-fix submitter.
• Companion test at capacity 1024 asserts the below-capacity path still requeues the batch, emits no drop warning, and leaves the drop counter untouched.
• Both counter assertions measure deltas and the subtests run serially, since the metrics are package-level and shared with the existing over-capacity subtests.
• go test ./controlplane/telemetry/... passes except internal/netns, which fails identically on main in this environment (needs privileges).