## Motivation

Closes linkerd#3916

This adds the ability to get profiles for services by IP address.

### Change in behavior

When the destination server receives a `GetProfile` request with an IP address,
it now tries to map that IP address to a service.

If the IP address maps to an existing service, then the destination server
returns the profile stream subscribes for updates to the _service_--this is the
existing behavior. If the IP changes to a new service, the stream will still
send updates for the first service the IP address corresponded to since that is
what it is subscribed to.

If the IP address does not map to an existing service, then the destination
server returns the profile stream but does not subscribe for updates. The stream
will receive one update, the default profile.

### Solution

This change uses the `IPWatcher` within the destination server to check for what
services an IP address correspond to. By adding a new method `GetSvc` to
`IPWatcher`, the server now calls this method if `GetProfile` receives a request
with an IP address.

## Testing

Install linkerd on a cluster and get the cluster IP of any service:

```bash
❯ kubectl get -n linkerd svc/linkerd-tap -o wide
NAME          TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)            AGE   SELECTOR
linkerd-tap   ClusterIP   10.104.57.90   <none>        8088/TCP,443/TCP   16h   linkerd.io/control-plane-component=tap
```

Run the destination server:

```bash
❯ go run controller/cmd/main.go destination -kubeconfig ~/.kube/config
```

Get the profile for the tap service by IP address:

```bash
❯ go run controller/script/destination-client/main.go -method getProfile -path 10.104.57.90:8088
INFO[0000] retry_budget:{retry_ratio:0.2  min_retries_per_second:10  ttl:{seconds:10}}
INFO[0000]
```

Get the profile for an IP address that does not correspond to a service:

```bash
❯ go run controller/script/destination-client/main.go -method getProfile -path 10.256.0.1:8088
INFO[0000] retry_budget:{retry_ratio:0.2  min_retries_per_second:10  ttl:{seconds:10}}
INFO[0000]
```

You can add and remove settings for the service profile for tap and get updates.

Signed-off-by: Kevin Leimkuhler <[email protected]>
Signed-off-by: Eric Solomon <[email protected]>

The proxy performs endpoint discovery for unnamed services, but not
service profiles.

The destination controller and proxy have been updated to support
lookups for unnamed services in #4727 and
linkerd/linkerd2-proxy#626, respectively.

This change modifies the injection template so that the
`proxy.destinationGetNetworks` configuration enables profile
discovery for all networks on which endpoint discovery is permitted.

…#4960)

The proxy performs endpoint discovery for unnamed services, but not
service profiles.

The destination controller and proxy have been updated to support
lookups for unnamed services in #4727 and
linkerd/linkerd2-proxy#626, respectively.

This change modifies the injection template so that the
`proxy.destinationGetNetworks` configuration enables profile
discovery for all networks on which endpoint discovery is permitted.