bgplgd add metrics api endpoint for open metrics

[Claudio Jeker]

This diff adds the required bits to export the bgpctl show metrics output
as openmetrics. For this /metrics needs to answer with a different
content-type. Apart from that adding this is trivial.

-- 
:wq Claudio

Index: bgplgd.c
===================================================================
RCS file: /cvs/src/usr.sbin/bgplgd/bgplgd.c,v
retrieving revision 1.1
diff -u -p -r1.1 bgplgd.c
--- bgplgd.c    28 Jun 2022 16:11:30 -0000      1.1
+++ bgplgd.c    28 Sep 2022 12:34:23 -0000
@@ -26,12 +26,15 @@
 #include "bgplgd.h"
 
 #define NCMDARGS       4
+#define OMETRIC_TYPE   \
+           "application/openmetrics-text; version=1.0.0; charset=utf-8"
 
 const struct cmd {
        const char      *path;
        char            *args[NCMDARGS];
        unsigned int    qs_mask;
        int             barenbr;
+       const char      *content_type;
 } cmds[] = {
        { "/interfaces", { "show", "interfaces", NULL }, 0 },
        { "/memory", { "show", "rib", "memory", NULL }, 0 },
@@ -41,6 +44,7 @@ const struct cmd {
        { "/rtr", { "show", "rtr", NULL }, 0 },
        { "/sets", { "show", "sets", NULL }, 0 },
        { "/summary", { "show", NULL }, 0 },
+       { "/metrics", { "show", "metric", NULL }, 0, 0, OMETRIC_TYPE },
        { NULL }
 };
 
@@ -107,7 +111,10 @@ bgpctl_call(struct lg_ctx *ctx)
        signal(SIGPIPE, SIG_DFL);
 
        /* Write server header first */
-       printf("Content-type: application/json\r\n\r\n");
+       if (ctx->command->content_type == NULL)
+               printf("Content-type: application/json\r\n\r\n");
+       else
+               printf("Content-type: %s\r\n\r\n", ctx->command->content_type);
        fflush(stdout);
 
        execvp(bgpctlpath, argv);