Allow to define grafana-agent resource requests in Loki chart values

[elcomtik]

Is your feature request related to a problem? Please describe.
Currently, we cannot define resource requests and limits for the Grafana agent deployed by the Loki chart

Describe the solution you'd like
Grafana agent CRD defines this property https://github.com/grafana/agent/blob/main/production/operator/crds/monitoring.grafana.com_grafanaagents.yaml, so we should be able to add this to Grafana agent template

loki/production/helm/loki/templates/monitoring/grafana-agent.yaml

Lines 16 to 30 in a3741d1

	spec:
	serviceAccountName: {{ include "loki.fullname" $ }}-grafana-agent
	enableConfigReadAPI: {{ .enableConfigReadAPI }}
	{{- include "grafana-agent.priorityClassName" $ | nindent 2 }}
	logs:
	instanceSelector:
	matchLabels:
	{{- include "loki.selectorLabels" $ | nindent 8 }}
	{{- with $.Values.monitoring.serviceMonitor}}
	{{- if .metricsInstance.remoteWrite}}
	metrics:
	instanceSelector:
	matchLabels:
	{{- include "loki.selectorLabels" $ | nindent 8 }}
	{{- end }}

Describe alternatives you've considered
The alternative is to disable self-monitoring to prevent the deployment of the Grafana agent by this chart.

Additional context

[elcomtik]

related to #10575