Add Coveralls support

Patch adds an additional job to Github Actions that runs tests with
enabled code coverage and send information about coverage to Coveralls,
see documentation [1]. Coveralls provides a convenient UI for analysis
code coverage [2].

1. https://docs.coveralls.io/go
2. https://coveralls.io/github/tarantool/go-tarantool

Author: ligurio

.github/workflows/testing.yml

@@ -24,6 +24,10 @@ jobs:
           - '1.10'
           - '2.8'
           - '2.9'
+        coveralls: [false]
+        include:
+          - tarantool: '2.9'
+            coveralls: true
 
     steps:
       - name: Clone the connector
@@ -44,3 +48,12 @@ jobs:
 
       - name: Run tests
         run: make test
+
+      - name: Run tests, collect code coverage data and send to Coveralls
+        if: ${{ matrix.coveralls }}
+        env:
+          COVERALLS_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          go get golang.org/x/tools/cmd/cover
+          go get github.com/mattn/goveralls
+          make coveralls

Makefile

@@ -1,4 +1,5 @@
 SHELL := /bin/bash
+COVERAGE_FILE := coverage.out
 
 .PHONY: clean
 clean:
@@ -12,3 +13,12 @@ deps: clean
 test:
 	go clean -testcache
 	go test ./... -v -p 1	
+
+.PHONY: coverage
+coverage:
+	go clean -testcache
+	go test ./... -v -p 1 -covermode=count -coverprofile=$(COVERAGE_FILE)
+
+.PHONY: coveralls
+coveralls: coverage
+	goveralls -coverprofile=$(COVERAGE_FILE) -service=github

README.md

@@ -5,6 +5,8 @@
 	<img src="https://travis-ci.org/tarantool/go-tarantool.png?branch=master" align="right">
 </a-->
 
+[![Coverage Status](https://coveralls.io/repos/github/tarantool/go-tarantool/badge.svg?branch=master)](https://coveralls.io/github/tarantool/go-tarantool?branch=master)
+
 # Client in Go for Tarantool 1.6+
 
 The `go-tarantool` package has everything necessary for interfacing with