resource control: improve observability for Controller and Server modules

## Enhancement Task

Improve the observability of Resource Control related modules on both the client-side Controller (`client/resource_group/controller`) and the server-side Resource Manager (`pkg/mcs/resourcemanager/server`). The current metrics and logging cover aggregate consumption well but lack visibility into allocation decisions, per-client behavior, and internal token state — all of which are critical for debugging resource contention and fairness in multi-tenant deployments.

### Motivation

- **Operators** cannot easily diagnose why a specific resource group or client is being throttled.
- **Developers** lack insight into how the demand-aware slot allocation algorithm distributes fill rates across clients.
- Several important internal states (limiter token balance, per-slot fill rate, RU demand samples, token depletion rate) are only logged at debug level or not exposed at all.

### Sub-Issues

- [ ] #10581 — Add client-side pre-throttling demand RU/s metric

### Scope

#### 1. Client-Side Controller Improvements

| Area | Current State | Proposed Improvement |
|---|---|---|
| **Token consumption per request** | `TokenConsumedHistogram` only observed in trace mode | Always observe token consumption; break down by RU type (read/write/SQL) |
| **Token depletion rate** | Only "low token notify" counter exists | Add metrics for token consumption rate vs. refill rate to detect imbalance before throttling |
| **Limiter state visibility** | Only active/tombstone status gauge | Expose current token balance, effective fill rate, and burst limit per resource group |
| **RU calculation breakdown** | Consumption sent to server as aggregate | Add per-component (KV CPU, read/write bytes, SQL CPU) consumption metrics on the client side |
| **Request wait time** | Only failed-request wait time histogram | Add wait time histogram for successful requests to understand latency impact of rate limiting |

#### 2. Server-Side Resource Manager Improvements

| Area | Current State | Proposed Improvement |
|---|---|---|
| **Per-slot fill rate allocation** | Calculated in memory, only debug-level logging | Add metrics exposing allocated fill rate per client/slot within a resource group |
| **Per-slot token grants** | `"assign slot tokens"` debug log only | Add histogram for token quantities granted per request to each client |
| **RU demand sampling** | Tracked internally in `ruTracker.sample()`, not exposed | Add metrics for sampled RU/sec per client to show which clients drive allocation decisions |
| **Slot lifecycle** | Logged at debug level only | Add gauge for active slot count per resource group and counter for slot creation/deletion |
| **Token loan/trickle time** | Calculated in `assignSlotTokens()`, only debug-level logging | Add metrics for loan amount and trickle duration to understand allocation smoothness |
| **Service limit impact** | Override metrics exist, but no cause tracking | Add metrics distinguishing throttling caused by service limit vs. configured fill rate |

### Non-Goals

- Changing the allocation algorithm itself.
- Adding new gRPC APIs or modifying the token bucket protocol.

### Related Code

- `client/resource_group/controller/` — Controller, group cost controller, limiter, metrics
- `pkg/mcs/resourcemanager/server/` — Manager, token buckets (slot allocation), keyspace manager, service limiter, metrics