This commit adds http client load-balancing with DNS discovery. For both A and SRV records.
It helps to route HTTP requests evenly for across discovered backends.
Discovered IP addresses are cached locally.
The main benefit of this feature is to remove intermediate vmauth as a load-balancer between
vmagent (vmalert) and remote targets. Which simplifies components management and
reduces operational overhead.
By default, it's disable and could be used with `dns+` or `srv+` hostname suffix at url.
For example, `-remoteWrite.url=http://dns+victoria-metrics:8428/api/v1/write`.
Fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2388
- Move lib/httputil.Transport to lib/promauth.NewTLSTransport. Remove the first arg to this function (URL),
since it has zero relation to the created transport.
- Move lib/httputil.TLSConfig to lib/promauth.NewTLSConfig. Re-use the existing functionality
from lib/promauth.Config for creating TLS config. This enables the following features:
- Ability to load key, cert and CA files from http urls.
- Ability to change the key, cert and CA files without the need to restart the service.
It automatically re-loads the new files after they change.
