MacBook Pro (mid-2012) Microphone Input
All PowerBook and MacBook models support analog audio in via the headphone jack.
In your specific case, it is possible to use external microphone on MacBook pro and Air 2012 Models.
It looks like Apple has redesigned it to be a multifunction plug, without telling us in plain English. What they do say is you can use the Apple iPhone headset with microphone! Now that is a Hint !

Since there are multiple options to connect a microphone lets take a look.
1 - For stand alone microphone with cable you could use a USB connector type or Audio to USB converter.
2- You could use a BlueTooth type microphone/Headset.
3 - And last not least, you could actually use your Headphone (as marked) the 1/8 inch plug, but NOT with standard audio plug.
Here is the how: The plug is like the headphone port on iOS devices, this jack supports audio input as well as output—with as described in Apple specification.
Looking under the hood of Apple &quot;About this Mac&quot; you get the answer (it says it does support it).

The microphone or headset in question must be of the three-ring variety, just like Apple’s headsets. The fourth pin (ring) is for the microphone !

OK, what now? Lets assume you insist on using your microphone!!!
Get the 3.5mm 4 Position to 2x 3 Position 3.5mm Headset Splitter Adapter - M/F

... this headset splitter adapter breaks the audio port out into two distinct ports - one that can be used for a microphone input connection, and the other for connecting external (output) speakers.
I found stores in Europe (for about €10), but sure you can find it where you are.