[Metrics][Discover] Render legacy histograms with a warning

[kpatticha]

closes #244194

Summary

• Support legacy histograms in the metrics experience (include them in the grid and render as 95th percentile via TO_TDIGEST in ES|QL).
• Add a generic user messages module and use it for legacy histogram messaging.

Changes

Legacy histogram support

• isLegacyHistogram helper (src/common/utils/legacy_histogram.ts): Returns true when both field type and instrument are 'histogram' (legacy histogram). Used for filtering, aggregation choice, and when to show user messages.
• use_metric_fields (src/components/observability/metrics/hooks/use_metric_fields.ts): Skips a metric only when it is a histogram field but not a legacy histogram (metricField.type === 'histogram' && !isLegacyHistogram(metricField)). Legacy histograms are now included in the grid.
• ES|QL aggregation (src/common/utils/esql/create_aggregation.ts): For legacy histogram, getAggregationTemplate returns PERCENTILE(TO_TDIGEST(??placeholder), 95) so the chart renders the 95th percentile. Other types (exponential_histogram, tdigest, counter, default) are unchanged.

Consumer-controlled user messages

• MetricsGrid (src/components/observability/metrics/metrics_grid.tsx): Accepts only getUserMessages?: (metric: MetricField) => UserMessage[] | undefined. No userMessages prop. For each chart it passes userMessages={getUserMessages ? getUserMessages(metric) : undefined}. The shared component no longer branches on metric type; the consumer decides what to pass.
• Metrics experience consumer (src/components/observability/metrics/metrics_experience_grid_content.tsx): Passes getUserMessages that returns LEGACY_HISTOGRAM_USER_MESSAGES when isLegacyHistogram(metric) is true, otherwise undefined. So messages are shown only for legacy histogram metrics, and the decision lives in the consumer.

User messages module

• Generic user messages (src/common/utils/user_messages.ts): New module that exports LEGACY_HISTOGRAM_USER_MESSAGES (warning about T-Digest encoding). Re-exported from src/common/utils/index.ts. Other message sets can be added to this module later.

Tests

• legacy_histogram.test.ts: Unit tests for isLegacyHistogram (true when type + instrument both histogram; false for histogram + other instrument, histogram + undefined, non-histogram type).
• create_aggregation.test.ts: Tests for getAggregationTemplate including legacy histogram (returns PERCENTILE(TO_TDIGEST(...), 95)) and non-legacy histogram (returns AVG(...)). Existing tests for replaceFunctionParams and createTimeBucketAggregation remain.
• metrics_grid.test.tsx: Removed the test that expected a userMessages prop. Kept the test that verifies getUserMessages(metric) is called per chart and its return value is passed to each chart.

How to test

• ./forge --dataset histograms --count 5 --interval 10s --purge
• check discover

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[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 316e50e

Failed CI Steps

• Jest Tests #2

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
discover	2081	2083	+2

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
discover	1.6MB	1.6MB	+816.0B

History

• 💔 Build #398471 failed 0759a8a

[lucaslopezf]

Nice!! Could we add a test here for the legacy histogram, since we have for the other ones?

[kpatticha]

Nice!! Could we add a test here for the legacy histogram, since we have for the other ones?

done: 326b36c