Add Email Metrics API docs partial

[wstrinz]

TL;DR

Adds public API docs for the email-metrics read endpoint (GET /v2/:account_id/metrics/email) to developer.drip.com. Companion to the drip monolith GA (drip#31360, which removes the metrics_api feature flag). Docs-only; no site config or dependency changes.

Background

The email-metrics endpoint is being GA'd (flag removal in drip#31360). It has no public doc partial today, so once GA'd it would be an undocumented endpoint. This is the hand-authored Slate partial for it. (developer.drip.com is this repo's Slate site, not spec-driven — so the doc is authored here, not generated from the monolith's OpenAPI YAML.)

Modification

• New source/includes/rest/_email_metrics.md — response shape (summary + per-email emails[] breakdown), the three currently-accepted params (start_date, end_date, broadcast_ids), the 20 req/hr/account rate limit + 429/Retry-After, and the 10-most-recent cap on emails[].
• source/index.html.md — one line adding rest/email_metrics to the nav list, after Campaigns (it reports on broadcast/campaign performance).

Field names, nesting, rounding (rates .round(4), revenue in cents), and the broadcast_id-only-for-broadcasts conditionality were verified against the monolith jbuilders and independently re-verified in a non-maker review. sort_by/page/per_page are intentionally not documented — the controller doesn't accept them yet (tracked separately).

Result

An "Email Metrics" section on developer.drip.com documenting the current surface accurately. cURL/shell sample only (no drip client-lib helper confirmed).

How to verify

Build the Slate site and view the Email Metrics section. NB: the local build currently fails on a pre-existing Ruby-3.x toolchain gap (webrick, then net/ftp missing from this Gemfile) — unrelated to this change; the same gap is being addressed by separate uncommitted Gemfile work. Content was verified statically (valid markdown/HTML, JSON example parses, valid nav YAML, same include mechanism as every live partial) and the response shape verified against the jbuilders.

Checklist

• Non-maker review (LGTM-with-nits; the one nit — the emails[] 10-cap note — is applied)
• Response shape verified against monolith jbuilders
• Local render confirmed (blocked by the pre-existing webrick/net-ftp toolchain gap — needs the Gemfile fix)
• Publish — Captain's call (the release step), coordinate with drip#31360 GA timing