* Add sidecar container support for linkerd-prometheus
Adds a new setting to the Prometheus' Helm config, allowing adding any kind of sidecar containers to the main container.
The specific use case that inspired this was for exporting data from Prometheus to external systems (e.g. cloudwatch, stackdriver, datadog) using a process that watches the prometheus write-ahead log (WAL).
Signed-off-by: Nathan J. Mehl <n@oden.io>
This moves Prometheus as a add-on, thus making it optional but enabled by default. The also make `linkerd-prometheus` more configurable, and allow it to have its own life-cycle for upgrades, configuration, etc.
This work will be followed by documentation that help users configure existing Prometheus to work with Linkerd.
**Changes Include:**
- moving prometheus manifests into a separate chart at `charts/add-ons/prometheus`, and adding it as a dependency to `linkerd2`
- implement the `addOn` interface to support the same with CLI.
- include configuration in `linkerd-config-addons`
**User Facing Changes:**
The default install experience does not change much but for users who have already configured Prometheus differently, would need to apply the same using the new configuration fields present in chart README