MINOR: filters: Export the name of known filters

It could be useful to know if some filter is declared on a proxy or if it is
enabled on a stream.
diff --git a/include/proto/filters.h b/include/proto/filters.h
index 346d8a0..18cbddb 100644
--- a/include/proto/filters.h
+++ b/include/proto/filters.h
@@ -29,6 +29,11 @@
 
 #include <proto/channel.h>
 
+extern const char *trace_flt_id;
+extern const char *http_comp_flt_id;
+extern const char *cache_store_flt_id;
+extern const char *spoe_filter_id;
+
 #define FLT_ID(flt)   (flt)->config->id
 #define FLT_CONF(flt) (flt)->config->conf
 #define FLT_OPS(flt)  (flt)->config->ops
diff --git a/src/cache.c b/src/cache.c
index d52fcf5..834efc1 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -43,7 +43,7 @@
 					* messages (legacy implementation) */
 #define CACHE_F_HTX         0x00000002 /* The cache is used to store HTX messages */
 
-static const char *cache_store_flt_id = "cache store filter";
+const char *cache_store_flt_id = "cache store filter";
 
 struct applet http_cache_applet;
 
diff --git a/src/flt_http_comp.c b/src/flt_http_comp.c
index 0d3e52b..2d1a7fd 100644
--- a/src/flt_http_comp.c
+++ b/src/flt_http_comp.c
@@ -31,7 +31,7 @@
 #include <proto/sample.h>
 #include <proto/stream.h>
 
-static const char *http_comp_flt_id = "compression filter";
+const char *http_comp_flt_id = "compression filter";
 
 struct flt_ops comp_ops;
 
diff --git a/src/flt_trace.c b/src/flt_trace.c
index 92a51bf..0ccf1fc 100644
--- a/src/flt_trace.c
+++ b/src/flt_trace.c
@@ -33,6 +33,8 @@
 #include <proto/proto_http.h>
 #include <proto/stream.h>
 
+const char *trace_flt_id = "trace filter";
+
 struct flt_ops trace_ops;
 
 struct trace_config {
@@ -709,6 +711,7 @@
 			pos++;
 		}
 		*cur_arg = pos;
+		fconf->id   = trace_flt_id;
 		fconf->ops  = &trace_ops;
 	}