Previously, the pickDelegator function was not returning a
*hijackerDelegator so the return value did not implement the Hijacker
interface. As a result, code that attempts to hijack the connection
would fail when using a type assertion.
All the other cases returned the hijackerDelegator correctly.
The idea behind it is described in detail in
https://github.com/prometheus/client_golang/issues/320 .
This commit also updates the example given in
promhttp/instrument_server_test.go , which nicely illustrates the
benefit of this change.
So far, currying could be emulated by creating different metric vec's
with different values in their ConstLabels. This was quite difficult
to grasp - which is essentially what was done in the example mentioned
above. Now that this use case can be solved without ConstLabels, we
can safely declare ConstLabels as rarely used. (Perhaps we can
deprecate them entirely one day, but I'll take a raincheck on that
when the changes of v0.10 have materialized.) This commit thus also
updates the ConstLabel doc comments in the various Opts. (It contained
fairly outdated stuff anyway.)
As it turned out, it's not that esay to guess "common" combination of
interface upgrades. So I decided to just implement all 32 possible
combination of interface upgrades. (Only 16 with Go 1.7 and earlier.)
Clearly, this calls for code generation. But right now, we still need
to find out what's the best form of the code. For later additions,
implementing code generation might be useful.
Note that newDelegator is called for each HTTP request. Thus, this
commit aims to make the upgrade selection quick. (After the type
checks, it's just directly accessing an element in a slice.)
This allows to finally get rid of the infamous injection hook in the
interface. The old SetMetricFamilyInjectionHook still exist as a
deprecated function but is now implemented with the new plumbing under
the hood.
Now that we have multiple Gatherer implementation, I renamed
push.Registry to push.FromGatherer.
This commit also improves the consistency checks, which happened as a
byproduct of the refactoring to allow checking in both the "merge
gatherer" Gatherers as well as in the normal Registry.
To keep backwards compatibility while not creating circular import
chains, some code had to be duplicated. But all functions using it
have been declared deprecated hereby.
The new ways of instrumenting handlers will all go into the new
package, and ultimately, the prometheus package itself will be
completely igorant of HTTP.
Torin Sandall