server: redirect Grafana + Prometheus output to seperate logfile

Created by: slimsag

This redirects Grafana and Prometheus log output to a seperate logfile, located at (host / container):

• ~/.sourcegraph/data/grafana.log / /var/opt/sourcegraph/grafana.log
• ~/.sourcegraph/data/prometheus.log / /var/opt/sourcegraph/prometheus.log

The main benefit of doing this is that we separate auxiliary service logs (which are verbose, and won't usually be helpful to us/customers) from app logs (which customers should look at / inspect, etc.)

Fixes #5221

Test plan:

Created by: codecov[bot]

Codecov Report

Merging #5232 into master will decrease coverage by <.01%. The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #5232      +/-   ##
=========================================
- Coverage   45.71%   45.7%   -0.01%     
=========================================
  Files         736     736              
  Lines       45372   45372              
  Branches     2606    2606              
=========================================
- Hits        20741   20739       -2     
- Misses      22634   22636       +2     
  Partials     1997    1997

Impacted Files	Coverage Δ
cmd/server/shared/shared.go	0% <0%> (ø)
cmd/frontend/graphqlbackend/repository.go	22.05% <0%> (-0.99%)