feat(eap): collect more cluster load signals for storage routing

[pbhandari]

get_cluster_loadinfo now returns a wider LoadInfo bag so we can graph hottest-replica load and pick thresholds for a later routing / dynamic-sampling strategy. This PR is probe + gauges only; nothing branches on the new fields yet.

Signals (all float, default -1.0):

• cluster_load — max 1m load / CPU, as before
• concurrent_queries — max system.metrics.Query (was count() on system.processes)
• cgroup_user_time_normalized
• block_in_flight_ops
• memory_tracking
• part_mutation

Replica aggregations are max (the hot node). Missing CH metrics and old Redis blobs become -1.0; treat that as not-loaded. Probe failure still increments get_cluster_loadinfo_failure and does not gauge.

One INTERNAL query per cache miss. Single-node vs clusterAllReplicas SQL is unchanged.

Follow-up: look at the gauges, set thresholds, then a strategy that actually reads these.

[sentry]

Bug: If get_cluster() fails, the except block references cluster_name before it's assigned, causing an UnboundLocalError and masking the original exception.
_{Severity: MEDIUM}

Suggested Fix

Initialize cluster_name to a default value like None before the try block. This ensures the variable is always defined when the except block is executed, preventing the UnboundLocalError.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: snuba/web/rpc/storage_routing/load_retriever.py#L169

Potential issue: In the `get_cluster_loadinfo` function, if
`get_cluster(storage_set_key)` raises an exception, such as `UndefinedClickhouseCluster`
due to a misconfiguration, the `cluster_name` variable is never assigned. The `except`
block then attempts to reference `cluster_name` when creating a metric tag. This will
raise an `UnboundLocalError`, which masks the original, more informative exception about
the cluster configuration problem, making debugging more difficult.

_{Did we get this right? 👍 / 👎 to inform future reviews.}