Pin Mappings from Arduino Metro M4 Express Board to SAMD51
Edit: Since writing my answer, I found this Google Sheet that does a much better job.

Arduino has some schematics on their website. Here's an image I used to build the following list below.

There are in this format: Arduino Pin | SAMD51 Pin | SAMD51 Description

Analog

A0  3   PA02/EINT2/AIN0/VOUT0
A1  14  PA05/I5/AIN5/VOUT1/SERCOM0.1
A2  15  PA06/I6/VREFC/AIN6/SERCOM0.2
A3  13  PA04/I4/VREFB/AIN4/SERCOM0.0
A4  11  PB08/I8/AIN2/SERCOM4.0
A5  12  PB09/I9/AIN3/SERCOM4.1


Digital

D0  44  PA23/I2C/I7/SERCOM3.1 +5/SOF/I2SFS1
D1  43  PA22/I2C/116/SERCOM3.0+5.1/I2SDI
D2  40  PB17/SERCOM5.1/I2SMCK0
D3  39  PB16/I0/SERCOM5.0/I2SCK0
D4  26  PB13/I13/SERCOM4.1/I2SMCK1
D5  27  PB14/I14/SERCOM4.2/PCC8
D6  28  PB15/I15/SERCOM4.3/PCC9
D7  25  PB12/I12/SERCOM4.0/I2SCK1
D8  42  PA21/I5/SERCOM3+5.3/I2SDO
D9  41  PA20/I4/SERCOM3+5.2/I2SFS0
D10 37  PA18/I2/SERCOM1+3.2
D11 38  PA19/I3/SERCOM1+3.3
D12 36  PA17/I2C/I1/SERCOM1.1+3.0
D13 35  PA16/I2C/I0/SERCOM1.0+3.1
GND     
AREF    4   PA03/I3/VREFA/AIN1
SDA 63  PB02/I2/AIN14/SERCOM5.0
SCL 64  PB03/I3/AIN15/SERCOM5.1


SPI

MOSI    29  PA12/I12/I2C/SERCOM2.0+4.1
SCK 30  PA13/I13/I2C/SERCOM2.1+4.0
MISO    31  PA14/I14/SERCOM2+4.2