Make the set of labels consistent within each metric exported by Linkerd

Nobody has claimed this yet.

## Feature Request

### What problem are you trying to solve?

Make the set of labels consistent within each metric exported by Linkerd.

Per the [Prometheus doc](https://prometheus.io/docs/instrumenting/writing_exporters/#labels), a single metric should always be exported with the same set of labels.

Prometheus' `client_golang` enforces this requirement in `prometheus.Registry`:
https://github.com/prometheus/client_golang/blob/673e4a177a5816096fb34777d8006fd138dbab2f/prometheus/desc.go#L35-L39
To put it another way, the set of metrics and labels exported by the Linkerd proxy would not be permitted in `client_golang`, for example:

```
response_total{
namespace="linkerd"
deployment="linkerd-controller",
}
response_total{
namespace="linkerd"
statefulset="linkerd-controller",
}
```

Additionally, if we want to answer the question: "What deployments are calling my service?", we need to query Prometheus like this:

```
response_total{
deployment!="",
dst_deployment="linkerd-controller",
dst_namespace="linkerd"
}
```

### How should the problem be solved?

Introduce new labels:
- `workload`
- `name`
- `dst_workload`
- `dst_name`

#### Current example

```
response_total{
deployment="linkerd-web",
dst_deployment="linkerd-controller",
}
```

#### Proposed example

```
response_total{
workload="deployment",
name="linkerd-web",
dst_workload="deployment",
dst_name="linkerd-controller",
}
```

### How would users interact with this feature?

This change would be transparent, provided existing labels were maintained. It would enable users to re-export and interact with Linkerd metrics in a Prometheus-standard way. For example, "What deployments are calling my service?" becomes:

```
response_total{
workload="deployment",
dst_workload="deployment",
dst_name="linkerd-controller",
dst_namespace="linkerd"
}
```

Relates to #4101.

/cc @adleong @grampelberg

The issue names no implementation files or tests; start by locating the Linkerd proxy metric export path and the definitions for response_total and related metrics. Compare label sets across workload kinds, then verify that each metric uses consistent workload, name, dst_workload, and dst_name labels while preserving existing labels.

Written by the indexing model from the issue text.