Please address the comments from this code review:

## Overall Comments
- In `Headers.Set`, you can avoid repeated `http.Header(h)` conversions and an unnecessary delete when no values are provided by early-returning on an empty `values` slice and reusing a local `hdr := http.Header(h)` alias.
- The new `Headers.Add` helper is a good addition, but its comment is a bit vague; consider clarifying that it appends values to existing header entries (mirroring `http.Header.Add`) rather than replacing them, to make its behavior immediately clear to readers.

## Individual Comments

### Comment 1
<location path="pkg/telemetry/phonehome/interceptor_test.go" line_range="181" />
<code_context>
 	rp := getGRPCRequestDetails(ctx, err, "ignored grpc method", "request")
 	s.Equal(http.StatusOK, rp.Code)
-	s.Equal([]string{"gateway", "user"}, rp.Headers.Get(userAgentHeaderKey))
+	s.Equal([]string{"user", "gateway"}, rp.Headers.Get(userAgentHeaderKey))
 	s.Nil(rp.UserID)
 	s.Equal("request", rp.GRPCReq)
</code_context>
<issue_to_address>
**suggestion (testing):** Add more test coverage for User-Agent combinations and edge cases

This assertion now covers the case where both HTTP and gateway User-Agents are present. To fully validate `getGRPCRequestDetails` behavior, please add tests for:

- Only HTTP User-Agent (no gRPC/gateway metadata)
- Only gRPC/gateway User-Agent (no HTTP header)
- Multiple metadata User-Agent values (verify ordering)
- Neither HTTP nor metadata User-Agent (no header set)

These will better protect against regressions in User-Agent construction across different request shapes.

Suggested implementation:

```golang
	rp := getGRPCRequestDetails(ctx, err, "ignored grpc method", "request")
	s.Equal(http.StatusOK, rp.Code)
	s.Equal([]string{"user", "gateway"}, rp.Headers.Get(userAgentHeaderKey))
	s.Nil(rp.UserID)
	s.Equal("request", rp.GRPCReq)
	s.Equal("/wrapped/http", rp.Path)

func TestGetGRPCRequestDetails_UserAgentVariants(t *testing.T) {
	type testCase struct {
		name string
		md   metadata.MD
		want []string
	}

	testCases := []testCase{
		{
			name: "only HTTP user agent (no gateway user agent)",
			md: metadata.Pairs(
				"grpcgateway-user-agent", "user",
			),
			want: []string{"user"},
		},
		{
			name: "only gateway user agent (no HTTP user agent)",
			md: metadata.Pairs(
				"user-agent", "gateway",
			),
			want: []string{"gateway"},
		},
		{
			name: "both HTTP and gateway user agents (HTTP first)",
			md: metadata.Pairs(
				"grpcgateway-user-agent", "user",
				"user-agent", "gateway",
			),
			want: []string{"user", "gateway"},
		},
		{
			name: "multiple HTTP and gateway user agents preserve ordering",
			md: metadata.Pairs(
				"grpcgateway-user-agent", "user1",
				"grpcgateway-user-agent", "user2",
				"user-agent", "gateway1",
				"user-agent", "gateway2",
			),
			// Expect all HTTP user agents first, in order, then all gateway user agents, in order.
			want: []string{"user1", "user2", "gateway1", "gateway2"},
		},
		{
			name: "no HTTP or gateway user agent (header not set)",
			md:   metadata.MD{},
			want: nil,
		},
	}

	for _, tc := range testCases {
		tc := tc
		t.Run(tc.name, func(t *testing.T) {
			t.Parallel()

			ctx := metadata.NewIncomingContext(context.Background(), tc.md)
			rp := getGRPCRequestDetails(ctx, nil, "ignored grpc method", "request")

			got := rp.Headers.Get(userAgentHeaderKey)
			if tc.want == nil {
				require.Nil(t, got)
				return
			}

			require.Equal(t, tc.want, got)
		})
	}
}

```

To compile and run these tests, ensure the following in `pkg/telemetry/phonehome/interceptor_test.go`:

1. The import block includes:
   - `testing`
   - `context`
   - `github.com/stretchr/testify/require`
   - `google.golang.org/grpc/metadata`

   For example:
   ```go
   import (
       "context"
       "testing"

       "github.com/stretchr/testify/require"
       "google.golang.org/grpc/metadata"
   )
   ```

2. The metadata keys used here (`"grpcgateway-user-agent"` for the original HTTP User-Agent and `"user-agent"` for the gateway/gRPC client User-Agent) must match what `getGRPCRequestDetails` actually reads. If the function uses different metadata keys, update the `metadata.Pairs` calls accordingly.

3. The expected ordering in the `"multiple HTTP and gateway user agents preserve ordering"` test (`HTTP UAs first, then gateway UAs`) should reflect the logic in `getGRPCRequestDetails`. If the function orders them differently, adjust the `want` slice to match the intended behavior.
</issue_to_address>