[ravi-pplx]

The ecr strategy has no metrics, so a rollout can't show how often manifests come from cache, how much blob traffic is redirected to S3, or why requests fail. Some failures are invisible without them: a manifest that fails digest verification has already answered 200, and the client only sees a short body.

This adds three counters, documented in the README:

• cachew.ecr.requests_total{kind, method, result}: kind is manifest, blob, or unknown for a path that names neither. result is hit or miss for manifests, redirect for blob GETs, metadata for blob HEADs, not_found, rate_limited, not_served (paths or repositories the proxy refuses), or failure.
• cachew.ecr.failures_total{kind, reason}: digest_mismatch, no_length, partial, rejected (ECR refused the proxy's token), authorization (no token could be minted or refreshed), bad_redirect, upstream_status, internal, or client_canceled when the client left first. Connection failures use the transport_* statuses the CodeArtifact metrics already report; the classifier moves to origin_transport.go as originTransportStatus so both strategies share it.
• cachew.ecr.auth_events_total{event}: reuse, refresh, forced_refresh and failure, the same events the codeartifact strategy records.

Every label value comes from a fixed set. Repository names and digests are never labels.

Each request carries a small observation in its context. Whichever layer finds the cause records it, and the first one recorded wins: serveBlob, the error writer, the transport, or the digest verifier in the handler's fill goroutine. When the request finishes, the outcome is classified from that cause plus the response status and the X-Cachew-Result header. The response wrapper that already hid the cache's expiry header (renamed to ecrResponse) now also keeps the status. The /v2/ version check isn't counted. Metrics are recorded with an uncancelable context, as in the CodeArtifact strategy.

Tests use a recording fake, as the codeartifact metrics tests do. They cover each result and every failure reason except internal through the real handler and transport.