prom: basic builtin prometheus stats exporter

This is a vpp builtin alternative, not a replacement, for the existing
vpp_prometheus_exporter.

The plugin works by registering with http_static as a url handler for
stats.prom and handles requests by scraping the stats segment in the
main thread. It will therefore consume vpp process cpu cycles.

By default the plugin is disabled. To enable, first start the http
static server an then use "prom enable" cli.

Type: feature

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: If6888e965d1b2361f6a5546586068213d37079d1
diff --git a/MAINTAINERS b/MAINTAINERS
index 9081a36..ec69873 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -779,6 +779,11 @@
 M:	Florin Coras <fcoras@cisco.com>
 F:	src/plugins/http
 
+Plugin - Prom
+I:	prom
+M:	Florin Coras <fcoras@cisco.com>
+F:	src/plugins/prom
+
 cJSON
 I:	cjson
 M:	Ole Troan <ot@cisco.com>