Fixes an interface type conversion bug (#24)

* Fixes string conversion bug

This upstream commit broke interface type conversions:
gosnmp/gosnmp#264
The underlying value of the interface was changed from string to a byte
slice.

* Small changes to make the Go linter happy

Collects broadcast packet metrics (#22)

The original DISCOv1 used to collect both broadcast and multicast
metrics, but for some reason I chose not to collect those for DISCOv2.
Upon closer inspection, we decided that while multicast packet metrics
were indeed not necessary, that broadcast packet metrics could possible
be useful. This commit adds collection for ingress and egress broadcast
packets.

Fixes a typo of metric name for ifHCInOctets (#21)

* Fixes a typo of metric name for ifHCInOctets

Resolves issue #20.

* Adds Go module support to repository

The import path for the gosnmp library needed to change, as the old one
was deprecated and no longer in use. This commit also updates the
Dockerfile to use Go 1.16 and alpine 3.15.

* Changes necessary to support Go modules

I meant for this commit to be part of the previous commit (@1def767).
The previous commit introduced go.mod and go.sum, and this commit
includes some necessary changes to support that. Namely, the gosnmp
library moved to a new repository. I also updated the Docker image to Go
1.16 and alpine 3.15.

* Fixes a Go error

golang/go#32479

* Uses fmt.Sprintf() instead of rune() for strconv

Sets ifAlias for uplink to actual ifAlias value on interface. (#17)

Adds ifAlias label to all exported metrics. (#16)

* Adds ifAlias label to all exported metrics.

Records raw OID counter in each sample. (#15)

Expect a string from gosnmp PDUs of type OctetString (#12)

* gosnmp now returns a string for PDU type OctetString

* Updates unit tests to use string type for OctetString PDU types.

Expect a string from gosnmp PDUs of type OctetString (#12)

* gosnmp now returns a string for PDU type OctetString

* Updates unit tests to use string type for OctetString PDU types.

Adds a comment warning about changing a variable. (#11)

* Adds a comment warning about changing a variable.

* Adds a warning note to the assignment of Timestamp.

Catches SIGTERM and writes all data to disk before exiting. (#8)

* Catches SIGTERM and writes all data to disk before exiting.

* Defers ticker.Stop()s at invocation time, and returns immediately after Write() on a SIGTERM.

* Adds back mainCancel() to the sigterm case statement.