Add separate counter for server ID 0 in katran TPR

Summary:
This diff introduces a new counter `error_server_id_zero` to separately track server ID 0 cases in katran's TPR (TCP Packet Router) module, distinguishing them from other types of bad server IDs.

## Background
Currently, server ID 0 is treated as a "bad ID" and counted under `error_bad_id`. However, server ID 0 represents a special case that requires different debugging and handling compared to actual corrupted/invalid server IDs.

## Changes Made
1. **New Counter Added**: `error_server_id_zero` added to:
   - `tcp_router_stats` struct in `TPRTypes.h`
   - BPF `stats` struct in `tcp_pkt_router_structs.h`
   - Output formatting in `operator<<`

2. **Stats Collection Updated**:
   - `TPRStatsPoller.cpp` - Generic stats collection
   - `MetaTcpPktRouter.cpp` - Meta-specific stats with timeseries
   - `tpr_app.cpp` - App-level diff calculation

3. **BPF Logic Updated**: Replaced `stat->error_bad_id++` with `stat->error_server_id_zero++` for **server ID 0 specific cases only**:
   - **Active handler**:
     - Parse header option where server_id == 0 (line 50)
     - Storage failure sending 0 to passive (line 105)
     - No server_id received in established connection (line 142)
   - **Passive handler**:
     - Received server_id == 0 from peer (line 58)

4. **Preserved existing `error_bad_id` behavior**: Still tracks legitimate bad ID cases including:
   - ID mismatches between header and storage (non-zero values)
   - Wrong server_id during packet routing (misrouted packets)
   - Storage inconsistencies with non-zero IDs

5. **Test Updates**: Updated `TcpPktRouterIntegrationTest.cpp` to reflect new counter behavior:
   - Rogue sock_ops scenarios now expect `error_server_id_zero > 0`
   - Normal scenarios expect both counters to be 0
   - All 27 tests now pass

## Key Distinction
- **`error_server_id_zero`**: Counts cases where server_id is actually 0 (special case for debugging)
- **`error_bad_id`**: Counts cases where server_id is corrupted, mismatched, or invalid (actual errors)

## Benefits
- **Better observability**: Distinguish expected server ID 0 cases from unexpected bad IDs
- **Improved debugging**: Separate counters for different error conditions
- **Backward compatibility**: Existing `error_bad_id` counter remains functional

## Testing
- Built successfully: `fbcode//katran/tpr:proxy_tpr_lib` and `fbcode//katran/facebook/cpr/tpr:meta_katran_tpr_lib`
- **All 27 integration tests pass**: `fbcode//katran/facebook/cpr/tpr/test:tcp_pkt_router_test`
- All code properly formatted and linted
- Both counters will be exported to monitoring systems for comprehensive tracking
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Reviewed By: pdubovitsky

Differential Revision: D80974570

fbshipit-source-id: 86e733d386c15f1fa461a5331c2f5f6eba0e7377

Author: Fei Chen

katran/tpr/TPRStatsPoller.cpp

@@ -36,6 +36,7 @@ void TPRStatsPoller::setStatsCounters(const tcp_router_stats& stats) {
   setCounter("conns_skipped", stats.conns_skipped);
   setCounter("no_tcp_opt_hdr", stats.no_tcp_opt_hdr);
   setCounter("error_bad_id", stats.error_bad_id);
+  setCounter("error_server_id_zero", stats.error_server_id_zero);
   setCounter("error_write_opt", stats.error_write_opt);
   setCounter("error_sys_calls", stats.error_sys_calls);
   setCounter("ignoring_due_to_kde", stats.ignoring_due_to_kde);
@@ -154,6 +155,7 @@ TPRStatsPoller::collectTPRStats(int numCpus) {
     aggregateStats.conns_skipped += stat.conns_skipped;
     aggregateStats.no_tcp_opt_hdr += stat.no_tcp_opt_hdr;
     aggregateStats.error_bad_id += stat.error_bad_id;
+    aggregateStats.error_server_id_zero += stat.error_server_id_zero;
     aggregateStats.error_write_opt += stat.error_write_opt;
     aggregateStats.error_sys_calls += stat.error_sys_calls;
     aggregateStats.ignoring_due_to_kde += stat.ignoring_due_to_kde;

katran/tpr/TPRTypes.h

@@ -42,6 +42,7 @@ struct tcp_router_stats {
   uint64_t conns_skipped{0};
   uint64_t no_tcp_opt_hdr{0};
   uint64_t error_bad_id{0};
+  uint64_t error_server_id_zero{0};
   uint64_t error_write_opt{0};
   uint64_t error_sys_calls{0};
   uint64_t ignoring_due_to_kde{0};
@@ -92,6 +93,7 @@ inline std::ostream& operator<<(std::ostream& os, const tcp_router_stats& s) {
      << " no_tcp_opt_hdr=" << s.no_tcp_opt_hdr
      << " ignoring_kde=" << s.ignoring_due_to_kde
      << " error_bad_id=" << s.error_bad_id
+     << " error_server_id_zero=" << s.error_server_id_zero
      << " error_write_opt=" << s.error_write_opt
      << " error_sys_calls=" << s.error_sys_calls
      << " legacy_server_opt=" << s.legacy_server_opt

katran/tpr/bpf/tcp_pkt_router_active_hdlr.h

@@ -47,7 +47,7 @@ static inline int handle_active_parse_hdr(
     return err;
   }
   if (!hdr_opt.server_id) {
-    stat->error_bad_id++;
+    stat->error_server_id_zero++;
     TPR_PRINT(skops, "active parsed empty server id from the option");
     return PASS;
   }
@@ -102,7 +102,7 @@ static int handle_active_write_hdr_opt(
     // while storing 'hdr_opt.server_id'. Send id == 0 so that the passive
     // side can try sending back proper id in the next round.
     hdr_opt.server_id = 0;
-    stat->error_bad_id++;
+    stat->error_server_id_zero++;
     TPR_PRINT(skops, "active failed to read server id from storage");
   } else {
     hdr_opt.server_id = *existing_id;
@@ -139,7 +139,7 @@ static inline int handle_active_estab(
   }
   if (_UNLIKELY(!hdr_opt.server_id)) {
     // Received tcp-hdr-opt but no server_id recv'ed from peer.
-    stat->error_bad_id++;
+    stat->error_server_id_zero++;
     // write a server_id 0 out to tell passive side to resend its server_id
     set_write_hdr_cb_flags(skops, stat);
     // try to read in the next round

katran/tpr/bpf/tcp_pkt_router_passive_hdlr.h

@@ -55,7 +55,7 @@ static inline int handle_passive_parse_hdr(
   }
   if (!hdr_opt.server_id) {
     // no server_id received from peer.
-    stat->error_bad_id++;
+    stat->error_server_id_zero++;
     TPR_PRINT(skops, "passive received 0 server id");
     return PASS;
   }

katran/tpr/bpf/tcp_pkt_router_structs.h

@@ -37,6 +37,7 @@ struct stats {
   __u64 conns_skipped;
   __u64 no_tcp_opt_hdr;
   __u64 error_bad_id;
+  __u64 error_server_id_zero;
   __u64 error_write_opt;
   __u64 error_sys_calls;
   __u64 ignoring_due_to_kde;