feat(slo): name the burning dimension in the alert, and cut the message down

[chris13524]

The burn-rate alert's job is to say which of sixty-nine chains is burning. It did not.

Before:

Prod - SLO fast burn — SLO rpc-chain-availability (objective: 99% of RPC requests served, per chain) spent 13.888888888888888% of its 30-day error budget in the last 1h, and is still over-spending as of the last 5m. The limit for this tier is 2% per 1h (14.4x the sustainable pace); at that rate the whole month of budget is gone in ~2.1 days. Any chain_id label on this instance names the chain that is burning. This is a budget alert, not an outage alert: the SLI may look fine on a 5-minute chart and still be on track to miss the objective. Check the SLIs row of the dashboard with the range set to 1h, then that chain's row under Chain RPC Router.

After:

Prod - rpc-chain-availability on chain eip155:5000 burned 13.9% of its 30-day error budget in 1h (tier limit 2%)

rpc-chain-availability on chain eip155:5000 burned 13.9% of its 30-day error budget in the last 1h, and is still burning as of the last 5m. Tier limit is 2% per 1h — 14.4x the sustainable pace, which exhausts the month in ~2.1 days. Objective: 99% of RPC requests served, per chain. Budget alert, not an outage: a 5-minute chart can look healthy while the month is still being missed. Dashboard: SLIs row at 1h, then that chain's row under Chain RPC Router.

What changed

dimension_template — a new opt that names the dimension inline in both fields,
right after the SLI. A Go template, because only the caller knows what its SLIs aggregate
by, and guarded ({{ if $labels.chain_id }}) because the same rule covers SLIs that carry
no such label:

dimension_template: '{{ if $labels.chain_id }} on chain {{ $labels.chain_id }}{{ end }}',

The old prose sentence could never have worked — a notification is read as a one-line
title first, and the answer was in the middle of a paragraph.

dimension_hint is kept but now defaults to empty and moves after the objective. A
consumer with several aggregation labels may still want a sentence; nobody needs it by
default. No consumer breaks — pay-core passes its own and will keep rendering it.

printf "%.1f" on $value, so alerts stop reporting 13.888888888888888%.

Subtraction — the message stated the tier arithmetic three ways and spent three lines
on "this is a budget alert, not an outage alert". That idea is worth one clause. ~600
chars → ~480, with the important part first instead of last.

Testing

render_rules.jsonnet renders and all three promtool suites pass. smoke.jsonnet updated
to the new opt.

Note

The printf is the one thing not verifiable offline — Grafana's $value is documented
as a string in some contexts, and the raw float in the "before" above suggests it is a
float64 here. If it turns out to be a string, that one substitution renders
%!f(string=...) and the rest of the message is unaffected. Worth an eyeball on the
first firing.