reverseproxy: Add duration/latency placeholders (close #4012) (#4013)

* reverseproxy: Add duration/latency placeholders (close #4012) (and #2268)

Adds 4 placeholders, one is actually outside reverse proxy though:

{http.request.duration} is how long since the server decoded the HTTP request (headers).
{http.reverse_proxy.upstream.latency} is how long it took a proxy upstream to write the response header.
{http.reverse_proxy.upstream.duration} is total time proxying to the upstream, including writing response body to client.
{http.reverse_proxy.duration} is total time spent proxying, including selecting an upstream and retries.

Obviously, most of these are only useful at the end of a request, like when writing response headers or logs.

See also: https://caddy.community/t/any-equivalent-of-request-time-and-upstream-header-time-from-nginx/11418

* Add new placeholders to documentation

1 files changed, 1 insertions, 0 deletions

diff --git a/modules/caddyhttp/app.go b/modules/caddyhttp/app.go
index 4f5bc84..8285200 100644
--- a/modules/caddyhttp/app.go
+++ b/modules/caddyhttp/app.go
@@ -49,6 +49,7 @@ func init() {
 // ------------|---------------
 // `{http.request.body}` | The request body (⚠️ inefficient; use only for debugging)
 // `{http.request.cookie.*}` | HTTP request cookie
+// `{http.request.duration}` | Time up to now spent handling the request (after decoding headers from client)
 // `{http.request.header.*}` | Specific request header field
 // `{http.request.host.labels.*}` | Request host labels (0-based from right); e.g. for foo.example.com: 0=com, 1=example, 2=foo
 // `{http.request.host}` | The host part of the request's Host header