Support semantically correct limit_ratio in ALB TSM fanout

Parent: #1068

Summary

Add semantically correct PromQL limit_ratio(r, v) support to the ALB Time
Series Merge (TSM) mechanism.

For a fixed ratio, Prometheus selects each input sample independently using a
deterministic hash of its complete label set. This selection is distributive
across shards: selecting on each shard and then unioning/deduplicating the
results is equivalent to selecting from the combined input vector. However,
the inner expression must itself have correct federated semantics. For example,
limit_ratio(0.1, sum by (service) (...)) must globally merge the inner sum
before applying limit_ratio.

Scope

• Support literal ratios in the full PromQL range [-1, 1], including the
  complementary selection semantics of negative ratios.
• Support both by (...) and without (...) grouping forms, before or after
  the aggregation arguments.
• Support instant and range queries.
• Preserve float and native-histogram samples selected by the operator.
• Preserve common outer sort and sort_desc wrappers with the same range
  query behavior as Prometheus.
• Remove the current inaccurate-result warning for supported expressions.
• Retain a clear fallback warning for parameter expressions or nested query
  shapes that the TSM planner cannot evaluate globally.

Implementation guidance

• Add a parsed aggregation specification for limit_ratio containing the
  ratio, inner query, grouping, and optional sort wrapper.
• Add a fast path for a fixed ratio over a shard-local inner expression:
  allow each Prometheus shard to apply limit_ratio, then use normal TSM
  union/deduplication.
• For an inner aggregation that TSM can merge, rewrite the request to the
  inner expression, perform its global TSM merge, and apply limit_ratio
  once in the finalizer.
• Make the finalizer use a Prometheus-compatible label hash and threshold:
  select hash/maxUint64 < r for non-negative r, and
  hash/maxUint64 >= 1+r for negative r.
• Hash the labels visible to PromQL after any configured TSM-only injected
  labels have been handled; do not let routing-only labels change the
  sample set.
• Treat unsupported or shard-dependent scalar parameter expressions
  conservatively. Do not silently evaluate different ratios per shard.
• Ensure partial fanout failure remains visible as an incomplete-global-
  result warning.

Acceptance criteria

• limit_ratio(r, v) returns the same label sets and values as a single
  global Prometheus-compatible query layer for supported expressions.
• Positive and negative complementary ratios produce complementary sets,
  modulo the documented boundary behavior.
• Ratios -1, 0, and 1 match Prometheus edge-case behavior.
• Results are deterministic across repeated requests and independent of
  ALB pool member ordering.
• Duplicate HA replicas do not cause duplicate output series.
• by and without grouping work for instant and range queries.
• Supported queries no longer include the current inaccurate-result
  warning.
• Unsupported parameter or nested-expression forms retain an explicit
  warning rather than returning silently inaccurate data.

Tests

• Parser tests for prefix/postfix grouping, whitespace, negative ratios,
  wrappers, malformed queries, and unsupported scalar expressions.
• Unit tests for the hash threshold and negative-ratio complement.
• Instant- and range-finalizer tests, including sparse timestamps and
  native histograms.
• Multi-shard tests with disjoint data, exact HA duplicates, unequal shard
  sizes, and reordered pool members.
• Nested merge test such as
  limit_ratio(0.5, sum by (service) (requests)).
• Partial-failure and capture-limit tests.

Prometheus references

• limit_ratio operator documentation
  defines the ratio range, deterministic pseudo-random selection, negative
  ratio complement, grouping behavior, and histogram support.
• PromQL aggregation evaluator (engine.go)
  contains HashRatioSampler, AddRatioSampleWithOffset, and the
  LIMIT_RATIO aggregation path. These are the compatibility reference for
  label hashing, threshold comparisons, ratio clamping, and warnings.
• Prometheus label-set implementation (labels.go)
  defines the Labels.Hash() operation used by HashRatioSampler.
• Original Prometheus implementation PR #12503
  documents the design rationale for deterministic hash sampling and negative
  complementary ratios.
• Prometheus feature-flag documentation
  describes the experimental PromQL feature gate that includes limit_ratio.

Non-goals

• Building a general distributed evaluator for arbitrary scalar expressions.
• Claiming correctness for an inner PromQL expression whose own cross-shard
  operations cannot be planned by TSM.