feat: Add endpoint returning tenant configured limits

[salvacorts]

What this PR does / why we need it:

This PR adds a new API endpoint GET /config/tenant/{tenant}/limits that returns the configured limits (merged with the defaults) of a tenant.

Note, this may be useful to implement the tenants limits page in the new operator UI

loki/pkg/ui/frontend/src/config/routes.tsx

Lines 94 to 98 in dbf2bef

	{
	path: "/tenants/limits",
	breadcrumb: "Limits",
	element: <ComingSoon />,
	},

[slim-bean]

I'm not sure if we have a root prefix for paths like this? given that this is now going to be something relied upon by other services I think the path should have a version in it, if that's not already provided by an additional prefix somewhere else

[salvacorts]

We don't add any prefix to that route. My concern with adding it under /loki/api/v1 is that I think we risk to somehow accidentally exposte the endpoint and make it public. I see this more like an operator endpoint similar to the /config or /ring ones.

I agree adding versioning here makes sense, what about /config/tenant/v1/{tenant}/limits?

[slim-bean]

I'm also wary about how this changes our multi-tenant paradigm, might it be better to not have a tenant path param and instead use headers the way we do multi-tenant in the rest of our API's?

[salvacorts]

yeah, good idea 👍 I will get the tenant from the header

[slim-bean]

I have a few questions with this approach, I think we should have a discussion before merging this

[salvacorts]

Pushed your suggestions Ed, this is how the endpoint looks like now

$ http 127.0.0.1:3100/config/tenant/v1/limits X-Scope-OrgID:12345
HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8
Date: Thu, 10 Apr 2025 12:55:44 GMT
Transfer-Encoding: chunked

ingestion_rate_strategy: global
ingestion_rate_mb: 8
ingestion_burst_size_mb: 9