Add `ate.imagecache.requests`: hit and miss telemetry for the node-local image cache

[JeffLuoo]

Summary

The node-local OCI image cache (internal/imagecache) emits no telemetry today.

The cache is on the Resume path: atelet calls Store.EnsureImage before it assembles the OCI bundle for an Actor. A hit costs no network I/O. A miss causes a pull and an unpack, which can take tens of seconds for a large image.

Metric

Field	Value
Name	ate.imagecache.requests
Instrument	Int64Counter
Unit	{request}
Description	Number of image lookups in the node-local image cache, by outcome.
Golden signal	Saturation (leading indicator for latency)
Emitted by	atelet (internal/imagecache, Store.EnsureImage)

Attributes

Key	Values	Notes
ate.imagecache.outcome	hit, miss	New key. A hit is a complete image record with all layer directories present. A miss is each other result.
error.type	standardized error values	Set it only when the lookup fails. Its absence means success.