kvcoord: Disable follower reads for TestProxyTracing

This commit disables follower reads for the test TestProxyTracing. We
noticed that sometimes, the test gets slow, and by the time we issue a
read request, it's served via follower reads instead of proxying it to
the leaseholder.

Fixes: #135493

Release note: None

pkg/kv/kvclient/kvcoord/dist_sender_server_test.go

@@ -4847,6 +4847,9 @@ func TestProxyTracing(t *testing.T) {
 		}
 		kvserver.RangefeedEnabled.Override(ctx, &st.SV, true)
 		kvserver.RangeFeedRefreshInterval.Override(ctx, &st.SV, 10*time.Millisecond)
+		// Disable follower reads to ensure that the request is proxied, and not
+		// answered locally due to follower reads.
+		kvserver.FollowerReadsEnabled.Override(ctx, &st.SV, false)
 		closedts.TargetDuration.Override(ctx, &st.SV, 10*time.Millisecond)
 		closedts.SideTransportCloseInterval.Override(ctx, &st.SV, 10*time.Millisecond)