feat(slack): Support unfurling Explore Metrics URLs in Slack

[DominikB2014]

Add Slack unfurl support for Explore Metrics URLs (/explore/metrics/),
alongside the existing traces and logs support.

The metrics URL structure differs from traces/logs — it uses a single metric
JSON query param with nested aggregateFields containing the yAxes, rather
than separate visualize or aggregateField params. The handler parses this
structure and uses the tracemetrics dataset for the events-timeseries API call.

Supports both /organizations/{slug}/explore/metrics/ and customer domain
{slug}.sentry.io/explore/metrics/ URL formats.

Depends on #112677

Fixes DAIN-1477

[linear-code]

DAIN-1477 Unfurl metrics explorer urls in slack

[DominikB2014]

@cursor review

[narsaynorath]

Nice. Just one comment about the wording of how to many metrics there are for clarity

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[sentry]

Bug: Metric parameter parsing and override logic runs for all explore URL types, not just metrics, potentially causing incorrect query and sort overrides for traces or logs URLs.
_{Severity: LOW}

Suggested Fix

Wrap the metric query and sort override logic in a conditional check to ensure it only executes when explore_dataset is SupportedTraceItemType.TRACEMETRICS. This will prevent the overrides from being applied to non-metric URL unfurls.

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: src/sentry/integrations/slack/unfurl/explore.py#L301-L304

Potential issue: The function `map_explore_query_args` unconditionally parses the
`metric` query parameter and applies its internal `query` and `aggregateSortBys` values.
This logic runs for all explore URL types, including traces and logs. If a traces or
logs URL contains a `metric` parameter (e.g., through URL manipulation or a frontend
bug), the `query` and `sort` values from the URL will be incorrectly overridden by the
values from the `metric` parameter. For example, a URL's `aggregateSort` parameter would
be ignored in favor of the `aggregateSortBys` from the `metric` JSON, leading to an
incorrect unfurl.

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