endpoint: Discover statefulset hostnames

Created by: keegancsmith

Pods in a stateful set have DNS addresses of the form podname.servicename. We can discover this by inspecting the endpoints from the headless service. They will have the Hostname field set on the endpoint address object. Note: only if podname.servicename is a valid address will hostname be set.

Additionally we support rpc:// scheme. When using the rpc scheme, we return address strings instead (ie hostname:port). This is what our rpc package expects.

This allows us to dynamically discover the pods in the indexed-search statefulset. This will allow kubernetes administrators to scale the indexed-search statefulset without needing to update the INDEXED_SEARCH_SERVERS configuration. Note: This commit does not update the default for that yet.

In future we could use this for gitservers as well (more testing required though, for example we don't want to use this if a gitserver has to be marked as ready).

Part of https://github.com/sourcegraph/sourcegraph/issues/5725

Created by: codecov[bot]

Codecov Report

Merging #6295 into master will increase coverage by 0.03%. The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #6295      +/-   ##
==========================================
+ Coverage   39.04%   39.08%   +0.03%     
==========================================
  Files        1187     1187              
  Lines       60692    60696       +4     
  Branches     5807     5807              
==========================================
+ Hits        23700    23723      +23     
+ Misses      34815    34796      -19     
  Partials     2177     2177

Impacted Files	Coverage Δ
internal/endpoint/endpoint.go	43.8% <42.85%> (+18.16%)
cmd/frontend/graphqlbackend/repository.go	21.84% <0%> (-0.98%)
internal/search/backend/horizontal.go	90.32% <0%> (+1.61%)