* feat(infra): improved support for in-process testing
* support for killing of a running server using simulated Ctrl-C
* support for overriding os.Stdin
* migrated many tests from the exe runner to in-process runner
* added required indirection when defining Envar() so we can later override it in tests
* refactored CLI runners by moving environment overrides to CLITestEnv
New flag `--enable-jaeger-collector` and the corresponding
`KOPIA_ENABLE_JAEGER_COLLECTOR` environment variable enables Jaeger
exporter, which by default sends OTEL traces to Jaeger collector on
http://localhost:14268/api/traces
To change this, use environment variables:
* `OTEL_EXPORTER_JAEGER_ENDPOINT`
* `OTEL_EXPORTER_JAEGER_USER`
* `OTEL_EXPORTER_JAEGER_PASSWORD`
When tracing is disabled, the impact on performance is negligible.
To see this in action:
1. Download latest Jaeger all-in-one from https://www.jaegertracing.io/download/
2. Run `jaeger-all-in-one` binary without any parameters.
3. Run `kopia --enable-jaeger-collector snapshot create ...`
4. Go to http://localhost:16686/search and search for traces
When enabled, metrics are pushed to the provided Prometheus Push
Gateway at the start and end of each command and periodically every
few seconds.
```
--metrics-push-addr=http://address:port
--metrics-push-interval=5s
--metrics-push-job=kopia
--metrics-push-grouping=a:b --metrics-push-grouping=c:d
--metrics-push-username=user
--metrics-push-password=pass
```
