helm: Add loadBalancerIP option to loki chart

[vhrosales]

Add loadBalancerIP to loki
The chart's service already has annotation options enabled
https://github.com/grafana/loki/blob/master/production/helm/loki/templates/service.yaml#L15

One of the usages of the annotation is to add an internal load balancer and some cloud providers allow you to select a static IP like:

  service:
    annotations:
      service.beta.kubernetes.io/azure-load-balancer-internal: "true"
    type: LoadBalancer
    loadBalancerIP: 10.2.17.3

Test

$ helm3 upgrade --install loki ./loki-stack-0.38.0.tgz -f values/staging/loki.yaml --namespace infra  
Release "loki" has been upgraded. Happy Helming!
NAME: loki
LAST DEPLOYED: Thu Jun 18 16:43:03 2020
NAMESPACE: infra
STATUS: deployed
REVISION: 2
NOTES:
The Loki stack has been deployed to your cluster. Loki can now be added as a datasource in Grafana.

See http://docs.grafana.org/features/datasources/loki/ for more detail.
$ kubectl -n infra get svc
NAME                            TYPE           CLUSTER-IP     EXTERNAL-IP   PORT(S)          AGE
loki                            LoadBalancer   10.0.124.174   10.2.17.3     3100:30990/TCP   61m
loki-headless                   ClusterIP      None           <none>        3100/TCP         61m
prometheus-kube-state-metrics   ClusterIP      10.0.199.224   <none>        8080/TCP         6d5h
prometheus-node-exporter        ClusterIP      None           <none>        9100/TCP         6d5h
prometheus-server               LoadBalancer   10.0.137.207   10.2.17.2     80:31265/TCP     6d5h

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #2238 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2238      +/-   ##
==========================================
- Coverage   62.04%   62.00%   -0.04%     
==========================================
  Files         156      156              
  Lines       12531    12531              
==========================================
- Hits         7775     7770       -5     
- Misses       4147     4150       +3     
- Partials      609      611       +2     

Impacted Files	Coverage Δ
pkg/promtail/targets/tailer.go	73.86% <0.00%> (-2.28%)	⬇️
pkg/promtail/targets/filetarget.go	68.67% <0.00%> (-1.81%)	⬇️

[cyriltovena]

LGTM