Add a runtime config handler

[annanay25]

Is your feature request related to a problem? Please describe.

I often want to access the tenant limits configured on a tempo instance through an HTTP API.

Describe the solution you'd like

Add a runtime config handler that provides information from the runtimeCfg manager.

Currently the runtimeCfg manager is implemented as a subservice of the Overrides module.

tempo/modules/overrides/overrides.go

Lines 61 to 71 in 5ad92c9

	runtimeCfg := runtimeconfig.ManagerConfig{
	LoadPath: defaults.PerTenantOverrideConfig,
	ReloadPeriod: time.Duration(defaults.PerTenantOverridePeriod),
	Loader: loadPerTenantOverrides,
	}
	runtimeCfgMgr, err := runtimeconfig.NewRuntimeConfigManager(runtimeCfg, prometheus.DefaultRegisterer)
	if err != nil {
	return nil, fmt.Errorf("failed to create runtime config manager %w", err)
	}
	tenantLimits = tenantLimitsFromRuntimeConfig(runtimeCfgMgr)
	subservices = append(subservices, runtimeCfgMgr)

We should split the runtimeCfg manager as a separate service and register an API endpoint for it. Cortex implementation for reference:
https://github.com/cortexproject/cortex/blob/fbf067776f612ec33e7228430d4670ef566b57ec/pkg/cortex/modules.go#L150-L178

Describe alternatives you've considered

Additional context