services/horizon/internal/ingest: Fix nil pointer dereference in captive core metrics

[tamirms]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

There is a bug where horizon panics with a nil dereference when running Horizon with remote captive core enabled.
The nil deref occurs on this line:

if !s.config.EnableCaptiveCore || (s.config.CaptiveCoreToml.HTTPPort == 0)

When remote captive core is enabled s.config.EnableCaptiveCore is true but s.config.CaptiveCoreToml is nil since the toml file is only fore local captive core.

Known limitations

[N/A]

[tsachiherman]

if s.config.CaptiveCoreToml could be a nil value, the better solution would be to test that explicitly instead of assuming the relationship between that and s.config.RemoteCaptiveCoreURL.

i.e.

if !s.config.EnableCaptiveCore || // captive core is disabled
   (s.config.CaptiveCoreToml != nil && s.config.CaptiveCoreToml.HTTPPort == 0) { // captive core http port is disabled
	return -1
}

[tamirms]

@tsachiherman @sreuland

EnableCaptiveCore implies we're either using a local captive core or a remote captive core.
s.config.RemoteCaptiveCoreURL != "" is only true if we're using a remote captive core.
s.config.CaptiveCoreToml is guaranteed to be non-nil if we're using a local captive core but it will be nil if we're not using captive core or we're using a remote captive core

[sreuland]

thanks for doing that additional refactor to clarify remote vs local captive core